Transforming growth factor-beta 1 is a pleiotropic peptide mediator of growth, differentiation, and extracellular matrix synthesis. In the embryonic chick heart prior to the formation of the endocardial cushions, evidence from in vitro experiments suggests that transforming growth factor-beta 1 may be an inducer of the differentiation of atrioventricular endothelial cells into endocardial cushion mesenchyme. Further in vitro evidence suggests that the factor stimulates mesenchymal cell proliferation, and, thus, growth of the cushions. Using an antibody made against a peptide duplicating the aminoterminal 30 amino acid sequence of transforming growth factor-beta 1, we stained sections of stage 11, 18, 23, 26, and 36 chick hearts by an in situ immunofluorescence technique. Transforming growth factor-beta 1 staining localized to the endocardial surface and epicardial surface of the stage 11 heart, but it decreased from these locations in later stages. The cardiac jelly (stage 11), endocardial cushions (stage 18, 23, and 26), and, subsequently, the heart valve leaflets (stage 36) stained intensely for the growth factor.