Peripheral blood lymphocytes obtained from cancer patients receiving interleukin-2 (IL-2) on two separate clinical protocols were evaluated for their in vitro responses to IL-2, alloantigens, and PHA. IL-2 in vivo induced enhanced in vitro proliferative responses to IL-2 and diminished in vitro proliferative responses to phytohemagglutinin (PHA) and alloantigens. Alloinduced cytotoxic T cell responses were also depressed following in vivo IL-2. We examined the kinetics of the in vitro proliferative response to PHA and IL-2 and found that while the response of lymphocytes primed in vivo with IL-2 to PHA was depressed at all times during the 2 week in vitro exposure, the response to IL-2 peaked earlier and higher than did the response to IL-2 by lymphocytes obtained prior to IL-2 therapy. These contrasting effects on antigen-induced T cell responses vs. IL-2 induced nonspecific proliferative and cytotoxic responses suggest the importance of dose and timing of IL-2 administration when used to enhance antigen-specific T cell responses or as an immune enhancing agent combined with vaccines.