21 CFR § 556.240 - Estradiol and related esters.
(b) Residues. Residues of estradiol are not permitted in excess of the following increments above the concentrations of estradiol naturally present in untreated animals:
(i) Muscle: 0.2 ppb.
(ii) Liver: 0.6 ppb.
(iii) Kidney: 1.2 ppb.
(iv) Fat: 1.2 ppb.