Human natural killer cell (NK cell) activity against K-562 target cell line was evaluated in full term cord blood (n = 30) and adult peripheral blood (n = 20) using 51Cr release assay. The level of NK cell activity was lower in cord blood compared to adult controls (39.6 +/- 11.4% vs 27.4 +/- 11.8% at effector:target ratio 50:1). Adult males showed a significantly higher NK activity compared to females. No sex difference was observed in cord blood. Furthermore, partially purified human leucocyte interferon (IFN alpha) increased in vitro NK cell function of both adult and newborn lymphocytes. The present results indicate that the appearance and maturation of human NK cells occurs during the intrauterine life of the human fetus.