The effect of human placental supernatant obtained from explant cultures of caesarean delivery placentae was monitored on both alpha-beta human T cell clones, which display both cytotoxic alloreactivity and non-MHC restricted cytotoxicity against K562 target cells, and gamma-delta ones endowed solely with the latter. It was found that, under appropriate experimental conditions, direct inhibition of the cytolytic activity of alpha-beta T cell clones was exerted by the supernatant. In contrast, gamma-delta T cell clones were unaffected. The relevance of these data to the survival of the fetal allograft is discussed.