Human chorionic gonadotrophin (HCG) and human chorionic somatomammotrophin (HCS) were shown to suppress the proliferative T and B cell response induced by different mitogens in human and mouse lymphocytes. Thyroid stimulating hormone (TSH), sharing the alpha-chain subunit with HCG was devoid of effect suggesting that the immunosuppressive entity is localized within the beta-chain subunit. Human growth hormone (HGH) which is partly biologically cross-reactive with HCS did not impair mitogen responsiveness. HCG was also able to inhibit the differentiation of cells since the induction of antibody-formation was depressed. Expression of plaque forming cells was also reduced by addition of HCG directly in the PFC assay possibly reflecting the anti-complementary effect of this hormone, since cell mediated lysis of target cells by Con A activated lymphocytes, which is not complement-dependent, was unaffected by adding HCG in the assay system.