Spermatozoa, obtained from 20 healthy individuals, were incubated with crude uterine fluid or with a purified IgG fraction obtained from uterine fluid before adding them to cultures with allogeneic lymphocytes. Untreated spermatozoa induced a proliferative 1.5- to 6-fold increase, while the spermatozoa treated with crude uterine fluid or with purified IgG fraction caused 84–100% suppression of lymphocyte blastogenesis. These results suggest the existence of an endogenous uterine fluid factor, apparently IgG, which causes inhibition of lymphocyte proliferation induced by spermatozoa in the uterine lume