We extracted human placentas by three methods to obtain chorionic TSH (hCT). The extracts were assayed by a sensitive homologous RIA for bovine TSH with an antibody which had veen used previously for assay of hCT. Based on the RIA for hCT, the yield of hCT was only 0.68 +/- 0.15 (SE) mU/100 g placenta by the Bates method, 2.01 +/- 0.35 mU/100 g by the Reisfeld method, and 4 mU/100 g by a Concanavalin A-Sepharose technique. Thyrotropic activity was detected by bioassay only in one Reisfeld extract; this biological activity could be attributed to hCG in this fraction. In TSH bioassays of other extracts, the responses were less than the sensitivity of the assay; some extracts were lethal to the bioassay mice. The low yield of hCT, about one-tenth to one-hundredth of that reported previously, casts doubt on the validity of the previous observations.