Natural killer (NK) cells can be differentiated into lymphokine-activated killer (LAK) effectors following stimulation with interleukin (IL)-2. This induction can be negatively regulated by IL-4. In this study, we demonstrate that the stimulation of NK cells through the CD2 pathway with (9-1 + 9.6) monoclonal antibodies can also induce these cells to secrete tumor necrosis factor-alpha (TNF-alpha) and to differentiate into LAK effectors. More importantly, our data indicate that, in contrast to the IL-2-induced LAK generation, the anti-CD2-triggered LAK activity was not regulated by IL-4. IL-4 was found to enhance the LAK activity as well as NK cell proliferation following activation with anti-CD2 by a mechanism involving, at least in part, an increased TNF-alpha production. Using immobilized monoclonal antibodies against the Fc receptor (Fc gamma RIII or CD16) for NK stimulation, we also observed that the anti-CD16-induced LAK activity was not inhibited by IL-4. These data further point to a pivotal role of TNF-alpha as a regulatory cytokine in anti-CD2-induced LAK generation, and suggest that IL-4 could serve as a discriminatory factor between two distinct pathways involved in the activation of non-MHC-restricted cytotoxicity.