The murine macrophage‐derived cell line P388D1 secretes the lymphokine inter‐leukin 1 (IL‐1) when stimulated by a variety of agents. When stimulated by bacterial lipopolysaccharide (LPS) the cells release IL‐1 in both high and low molecular weight (m.w.) forms. The proportion of high m.w. IL‐1 is reduced when IL‐1‐containing supernatants are concentrated by ammonium sulfate precipitation subsequent to hollow‐fiber filtration. The high m.w. form can be converted to the low m.w. form by proteolysis, reduction and alkylation, or chromatography in a dissociating solvent. The low m.w. form remains as such, even when reconcentrated in fetal calf serum‐containing medium. The high m.w. form thus likely consists of a complex between low m.w. IL‐1 and another protein secreted by the P388D1 cell line. Copyright © 1984 Alan R. Liss, Inc.