A hybrid cell line (DCH-5) constructed from an adherent cell of mouse spleen and the thymoma BW5147, and selected for adherence to hydrophobic surfaces, secretes a factor which augments the mitogenic response of thymocytes. Properties of the factor were compared with those of a P388D1 and J774.1 macrophage-derived interleukin 1 (IL-1). On polyacrylamide gel electrophoresis in Tris-glycinate buffer the IL-1-like factor of DCH-5 cells was heterogeneous and its components were more negatively charged than components of macrophage-derived IL-1. Charge differences between these factors were also confirmed by isoelectric focusing (IEF) (pI of IL-1 was 5.4, pI of the IL-1-like factor was 3.5). SDS-PAGE and gel filtration demonstrated that both factors consisted of several components of near molar masses (approximately 17 kg/mol). Gel filtration showed that in PBS the IL-1-like factor of DCH-5 cells was partially aggregated. Antibodies specific to IL-1 inhibited the activity of the IL-1-like factor of DCH-5 cells. Thus, mouse DCH-5 cells provide a new source of IL-1-like factor which might be useful for further elucidation of the heterogeneity of interleukins. © 1986.