The contractile mesangial cell has previously been shown to be a metabolically active cell capable of producing a variety of substances. In this communication, we report that the contractile mouse mesangial cell produces a factor which induces the replication of blood monocytes and splenic macrophages but not of peritoneal macrophages. This factor has an isoelectric pH of 4 as determined by chromatofocusing, a molecular weight of 68,000 daltons as determined by gel chromatography, and is reactive to anti-CSF-1 as assessed by experiments showing inhibition of function by the antibody. These physiological, physico-chemical and immunological characteristics indicate that the substance is most likely colony-stimulating factor-1 (CSF-1).