By means of cytotoxicity and thymidine uptake assays, the effect of human recombinant interleukin-2 (rIL-2) on the induction of lymphokine-activated killer (LAK) cell activity and cellular proliferation in splenocytes and thymocytes from human fetuses (18-22 weeks) was studied and compared with that in mononuclear cells from adult peripheral blood. It was shown that the fetal splenocytes and thymocytes incubated with low doses of rIL-2 (10-100 U/ml) developed a broad spectrum of antitumor activity (LAK cytotoxicity), although the kinetics and magnitudes of the responses were different. It was also suggested that LAK precursors were present in fetal spleen and thymus. Furthermore, rIL-2 induced a stronger proliferative and cytotoxic response in splenocytes than in thymocytes. The human fetal LAK cells isolated from the spleen were found to be 10-20 times more potent than those from adult peripheral blood with regard to cell proliferation and cytotoxicity in vitro.