Human interleukin-1 (IL-1), free of contaminating lymphokines, was isolated from cultures of purified monoblasts from a patient with acute monocytic leukemia. Partially purified IL-1 (diafiltration, ultrafiltration, and isoelectric focusing) stimulated proliferation of subconfluent human fibroblasts in vitro. Further purification of IL-1 by high-resolution gel filtration- and anion exchange-high performance liquid chromatography revealed that fibroblast proliferation activity could not be separated from IL-1 activity (thymocyte proliferation), suggesting that both activities are the properties of a single molecule. Fibroblasts and thymocytes exhibited a similar sensitivity to the proliferative effects of IL-1. These findings suggest that macrophages participating in inflammatory reactions in vivo might release IL-1, which could function to expand fibroblast populations at sites of inflammatory reactions, by acting as a fibroblast growth factor.