The effects of recombinant human interleukin-4 (rhu IL-4) on immunological parameters in patients receiving increasing doses of IL-4 in a Phase I trial were investigated. Peripheral blood mononuclear cells were phenotyped for a variety of lymphocyte markers, but no consistent effects were observed. However, increases in HLA Class II expression on monocytes were detected in four patients. NK and LAK activity were neither induced nor augmented by IL-4 treatment. Slight increases in proliferative responses to mitogens and cytokines were observed in some patients. The latter observations and other clinical studies suggest that a combination of IL-4 with IL-2 may be more effective in Phase II clinical trials.