The in vitro induction of LAK cell activity was studied in cancer and AIDS patients. F3, an immunoregulatory component of Astragalus membranaceus was shown capable of potentiating LAK cell activity induced by rIL-2. The LAK cells killing activity against Hs294T melanoma cell line induced by 50 U/ml rIL-2 in the presence of F3 (55 μg/ml) reached 64%, which was comparable to that (60%) induced by 500 U/ml of rIL-2 alone. With F3 and rIL-2, the effector to target ratio could be reduced to one-half in order to obtain an equivalent level of cytotoxicity induced by rIL-2 alone. In some patients whose peripheral blood lymphocytes were relatively inert of rIL-2, F3 could make them responsive to rlL-2 induction. These results imply that F3 may be useful to potentiate LAK cell activity, reduce the dosage of rIL-2 and thus minimize the later's toxic side effects when used in vivo.