The role of the T lymphocyte growth factor, interleukin 2 (IL2), on human lymphocyte proliferation was investigated. The results define the extent and the chronological sequence of IL2 effects on unstimulated mononuclear cells and on an activated population enriched in IL2 receptor positive cells. Important differences were observed in the proliferative response to the mitogen PHA when the IL2 concentration in the culture medium was varied. The data suggest that IL2 concentration is critical in the responsiveness to a mitogenic stimulus.