Lymphokine-activated killer (LAK) cell activity may be largely the result of activation and/or expansion of peripheral blood natural killer cells by culture with interleukin-2 (IL-2). We have examined the role of proliferation in LAK cell development by either inhibiting or enhancing the proliferative potential of peripheral blood lymphocytes. Inhibition of proliferation was accomplished using irradiation, mitomycin C, or the iron chelator deferoxamine. For each of these agents, a dose-dependent inhibition of proliferation was observed. At doses of inhibitor which nearly completely blocked thymidine uptake, the development of LAK activity was only partially impaired. The mitogenic lectins phytohemagglutinin (PHA) and concanavalin A (Con A) augmented the proliferative response of peripheral blood lymphocytes to IL-2. However, augmentation by PHA, but not Con A, consistently resulted in a decrease in LAK activity. This inhibition of LAK activity by PHA did not appear to be due to inhibition of the effector cell, nor to preferential expansion of irrelevant cells. These data suggests that not all LAK activity is dependent on proliferation, and that high levels of proliferation in the presence of IL-2 do not necessarily lead to LAK activity.