Murine peritoneal exudate macrophages incubated with medium conditioned by L929 cells were stimulated to produce lymphocyte-activating factor (LAF, Interleukin 1). This stimulatory activity was partially neutralized by antiserum prepared against partially purified L cell colony-stimulating factor (CSF) and comigrated upon gel filtration with the myeloproliferative activity. LAF-inducing activity of three different L cell CSF preparations, including one purified to homogeneity, was dependent upon the concentration of CSF. A minimum of 1,000 to 3,000 units of CSF activity was required to stimulate macrophagfes to produce LAF. Concanavalin A (Con A) stimulated splenic supernatants also contained CSF and LAF-inducing activities that co-eluted upon gel filtration. LAF-inducing activities co-eluting with the two Con A CSF peaks (apparent m.w. of 25,000 and 35,000) were effective at minimum dilutions containing 1,000 to 3,000 units of CSF activity correlating in potency with L cell-derived CSF. Based on these data, it is proposed that CSF, whether of L cell or lymphoid origin, not only has myeloproliferative activities but also is capable of stimulating macrophages to produce LAF.