Chronic lymphocytic leukemic B cells (B-CLL) were found to produce an IL 1-like growth factor spontaneously in vitro for mouse thymocytes. This factor was comitogenic with concanavalin A (Con A) and nonmitogenic combinations of phorbol ester and calcium ionophore but not with phyto-hemagglutinin (PHA). Growth factor production was dose-related to the number of in vitro cultured cells and detectable at 6 hours using high cell concentrations. A small number of admixed normal T cells was not important for factor production. No growth of autologous B-CLL or allogeneic thymocytes was induced by the factor. A chromatographic high-performance liquid chromatography (HPLC) analysis and inhibition experiments with a polyclonal rabbit anti interleukin 1 (IL 1) antiserum indicated that the B-CLL-derived growth factor belonged to the IL 1 family. This was supported by the direct demonstration of IL 1 beta in supernatants from B-CLL by radioimmunoassay. Possible biologic implications for B-CLL-derived IL 1 are discussed in relation to tumor cell growth in different clinical stages.