It has been previously reported that lipopolysaccharide-stimulated murine astrocytes produce a factor which enhances the proliferative response of thymocytes to lectins. The present report demonstrates that a rat astrocytoma cell (C6 cells)-derived factor appears to be similar to macrophage-derived interleukin 1 (IL 1) in its biological activities and biochemical characteristics. Upon injection into mice, supernatants of C6 cells induce the production of serum amyloid A. The C6 cell-derived factors enhance the response of thymocytes to phytohemagglutinin and the growth of fibroblasts whereas no effect on the growth of neuroblasts and of a strictly interleukin 2 (IL 2)-dependent T cell line was observed. On an AcA 54 column the C6-derived factors acting on thymocytes and fibroblasts coeluted as a single peak of Mr = 13 500 to 18 000; the semipurified factor was found to enhance the lymphocytes' production of IL 2. These observations demonstrate that cells not belonging to the mononuclear phagocyte lineage are able to produce factors identical or closely related to IL 1.