Despite administration of 3'-azido-3'-deoxythymidine (AZT, Zidovudine) to seriously immunocompromised patients, little has been reported regarding effects of AZT on specific immune functions. This study analyzed the in vitro effect of AZT on normal human lymphocyte cytolytic activity. AZT at concentrations up to 100 microM had no effect when added directly to cytotoxicity assays with lymphocyte effector cells and natural killer (NK)-sensitive or NK-resistant target cells. In contrast, addition of AZT to lymphocytes cultured for 4-10 days with interleukin-2 (IL-2) prior to cytotoxicity assays produced a concentration- and time-dependent inhibition; this effect was not mimicked by acyclovir or ganciclovir. Lymphocyte cell numbers and viability were not reduced in parallel to inhibition of cytolytic activity by AZT. Furthermore, AZT inhibition of IL-2-dependent cytolytic activity was not correlated with alterations in lymphocyte cell surface phenotypes by flow cytometry, and lymphocyte culture supernatant levels of interferon-gamma were not reduced by AZT. These results suggest that AZT may selectively inhibit human lymphocyte functions and thus may have implications for long-term therapeutic administration of AZT in chronic immunodeficiency states.