In vitro stimulation of mononuclear cells from human peripheral blood with mitogens causes the release of factors (monokines and lymphokines) which possess distinct biological activities. One such factor, termed 22K, can induce production of human beta-interferon (HuIFN-beta) in cultured human fibroblasts, thereby rendering these cells resistant to virus infection. Here we report the complete purification and partial sequencing (39 N-terminal amino acids) of this factor, whose relative molecular mass was estimated by SDS-polyacrylamide gel electrophoresis to be 17,000 (17K). In addition to an antiviral effect, the pure protein exhibits several other biological activities. Most significantly, intravenous (i.v.) injection of the factor in rabbits caused fever and granulopenia at doses of 0.1-1 microgram per kg, effects which we attribute to a monokine called endogenous pyrogen (EP). In vitro, the protein was scored as positive in a LAF (lymphocyte-activating factor) assay at 0.1-1 ng ml-1. LAF and EP are considered to be members of one family of monokines, called interleukin-1 (IL-1). For this reason, and also because the amino-acid sequence of the 22K factor is at least partially homologous to a complementary DNA-derived IL-1 sequence, we postulate that the 22K factor also belongs to the IL-1 family.