Sera of healthy pregnant women have a nonspecific inhibitory activity on the specific cytolytic activity of activated cytotoxic T lymphocytes in vitro. The serum fraction of pregnant females which is responsible for this activity has been characterized. The equivalent fraction derived from sera of healthy men has a similar but reduced activity compared with the female fraction. This suppressive factor seems to be activated during pregnancy. Moreover, this study sheds some light on the reasons behind the attenuation of the maternal immune system during pregnancy.