Activin possesses mesoderm-inducing activity, erythroid-differentiating activity, and follicle-stimulating hormone-releasing activity. The chemical structures of the activin molecule are formed by a combination of two β-subunit peptides of inhibin. Inhibin is a dimer consisting of an α and β subunit. To examine the mesoderm-inducing activity of these substances, we tested several configurations including : (1) two types of α-subunit peptide; (2) two types of inhibin A and B dimer; (3) β<sub>A</sub>-subunit peptide monomer; (4) three types of activins A, AB and B, and (5) follistatin (activin-binding protein) by the animal cap assay using Xenopus laevis ectoderm, and by the erythroid-differentiating factor (EDF) test. Activins, which are composed of dimeric inhibin β<sub>A</sub>-or β<sub>B</sub>-subunit peptides, had the highest mesoderm-inducing and EDF activities. The monomeric βA-subunit peptide exhibited mesoderm-inducing and EDF activities that were much lower than activin A. The inhibitory effect of follistatin on mesodermal induction by the β<sub>A</sub>-subunit peptide was also lower than that of activin. Both inhibins A and B had very weak mesoderm-inducing activity and no EDF activity. The two types of inhibin α-subunit monomer had little mesoderm-inducing activity and no EDF activity. The mesoderm induction caused by activin A was not suppressed by the addition of the α-subunit monomer and inhibin. The mesoderm-inducing activity in relation to the chemical structures of the monomeric and/or dimeric inhibin α and β<sub>A </sub>subunits is discussed.