AbstractUsing inununohistochemical methods we have investigated the presence of transforming growth factorsβ1,β2 andβ1 precursor in a two-week-old trilaminar human embryo. TGF-β1 precursor was seen in both the epiblast and the hypoblast. In contrast to the widespread localization of TGF-β1 precursor in the embryo proper, antibodies to the mature TGF-β1 peptide localized preferentially to the hypoblast with only weak staining in the epiblast. Staining with antibodies to TGF-β2 was generally weak in both the epiblast and hypoblast layers of the embryo proper. These results show that TGF-β1 andβ2 peptides are detectable as early as the second week of human development.