29 U.S. Code § 158 - Unfair labor practices
The expressing of any views, argument, or opinion, or the dissemination thereof, whether in written, printed, graphic, or visual form, shall not constitute or be evidence of an unfair labor practice under any of the provisions of this subchapter, if such expression contains no threat of reprisal or force or promise of benefit.
It shall be an unfair labor practice for any labor organization and any employer to enter into any contract or agreement, express or implied, whereby such employer ceases or refrains or agrees to cease or refrain from handling, using, selling, transporting or otherwise dealing in any of the products of any other employer, or to cease doing business with any other person, and any contract or agreement entered into heretofore or hereafter containing such an agreement shall be to such extent unenforcible  and void:
It shall not be an unfair labor practice under subsections (a) and (b) of this section for an employer engaged primarily in the building and construction industry to make an agreement covering employees engaged (or who, upon their employment, will be engaged) in the building and construction industry with a labor organization of which building and construction employees are members (not established, maintained, or assisted by any action defined in subsection (a) as an unfair labor practice) because (1) the majority status of such labor organization has not been established under the provisions of section 159 of this title prior to the making of such agreement, or (2) such agreement requires as a condition of employment, membership in such labor organization after the seventh day following the beginning of such employment or the effective date of the agreement, whichever is later, or (3) such agreement requires the employer to notify such labor organization of opportunities for employment with such employer, or gives such labor organization an opportunity to refer qualified applicants for such employment, or (4) such agreement specifies minimum training or experience qualifications for employment or provides for priority in opportunities for employment based upon length of service with such employer, in the industry or in the particular geographical area:
A labor organization before engaging in any strike, picketing, or other concerted refusal to work at any health care institution shall, not less than ten days prior to such action, notify the institution in writing and the Federal Mediation and Conciliation Service of that intention, except that in the case of bargaining for an initial agreement following certification or recognition the notice required by this subsection shall not be given until the expiration of the period specified in clause (B) of the last sentence of subsection (d). The notice shall state the date and time that such action will commence. The notice, once given, may be extended by the written agreement of both parties.
 So in original. Probably should be “unenforceable”.
1974—Subsec. (d). Pub. L. 93–360, § 1(c), (d), substituted “any notice” for “the sixty-day” and inserted “, or who engages in any strike within the appropriate period specified in subsection (g) of this section,” in loss-of-employee-status provision and inserted enumeration of modifications to this subsection which are to be applied whenever the collective bargaining involves employees of a health care institution.
Subsec. (g). Pub. L. 93–360, § 1(e), added subsec. (g).
1959—Subsec. (a)(3). Pub. L. 86–257, § 201(e), struck out “and has at the time the agreement was made or within the preceding twelve months received from the Board a notice of compliance with sections 159(f), (g), (h) of this title” after “such agreement when made” in cl. (i).
Subsec. (b)(4). Pub. L. 86–257, § 704(a), among other changes, substituted “induce or encourage any individual employed by any person engaged in commerce or in an industry affecting commerce to engage in, a strike or a refusal in the course of his employment” for “induce or encourage the employees of any employer to engage in, a strike or a concerted refusal in the course of their employment” in cl. (i), added cl. (ii), and inserted provisions relating to agreements prohibited by subsection (e) of this section in cl. (A), the proviso relating to primary strikes and primary picketing in cl. (B), and the last proviso relating to publicity.
Subsec. (b)(7). Pub. L. 86–257, § 704(c), added par. (7).
Subsec. (e). Pub. L. 86–257, § 704(b), added subsec. (e).
Subsec. (f). Pub. L. 86–257, § 705(a), added subsec. (f).
1951—Subsec. (a)(3). Act Oct. 22, 1951, substituted “and has at the time the agreement was made or within the preceding twelve months received from the Board a notice of compliance with section 159(f), (g), (h) of this title, and (ii) unless following an election held as provided in section 159(e) of this title within one year preceding the effective date of such agreement, the Board shall have certified that at least a majority of the employees eligible to vote in such election have voted to rescind the authority of such labor organization to make such an agreement:” for “; and (ii) if, following the most recent election held as provided in section 159(e) of this title the Board shall have certified that at least a majority of the employees eligible to vote in such election have voted to authorize such labor organization to make such an agreement:”.
1947—Act June 23, 1947, amended section generally by stating what were unfair labor practices by a union as well as by an employer, and by inserting provisions protecting the right of free speech for both employers and unions.
For effective date of amendment by act June 23, 1947, see section 104 of act June 23, 1947, set out as a note under section 151 of this title.
Section 705(b) of Pub. L. 86–257 provided that:
Act June 23, 1947, ch. 120, title I, § 102, 61 Stat. 152, provided that:
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