We studied the role of DNA synthesis in the induction of lymphokine-activated killer (LAK) cells by recombinant interleukin-2 (IL-2) and the dependence of this phenomenon on DNA synthesis. Doses of gamma-irradiation (1000-5000 rads) that profoundly reduced DNA synthesis in human peripheral blood mononuclear leucocytes (PBL) also effectively suppressed the development of cytotoxic activity in the absence of IL-2. However, the same doses of irradiation affected the induction of LAK activity by IL-2 to a much lesser extent. Blocking the formation of deoxyribonucleotides by hydroxyurea, which resulted in a complete inhibition of DNA synthesis in PBL or purified T lymphocytes, had virtually no effect on the generation of LAK cells. These results indicate that the expression of LAK activity is not dependent on DNA synthesis.