Human lung cancer cell line, T3M-30, has been shown to produce a growth factor that stimulates proliferation of peripheral blood monocytes. In the presence of this factor, human circulating monocytes were able to proliferate in vitro. Gel exclusion chromatography of the conditioned medium revealed a single peak of monocyte growth-promoting activity at an apparent molecular weight of 16,000. The growth-promoting activity was adsorbed to an anion-exchange column, Mono Q, and eluted with a salt gradient as a single peak of bioactivity at 300 mM NaCl. When the sample was applied to a Vydac C4 column, a reverse-phase high-performance liquid chromatography column, a single peak of activity was observed at a concentration of 76% acetonitrile in 0.1% trifluoroacetic acid. The monocyte growth-promoting activity was heat stable at 56 degrees C. It was partially destroyed by trypsin. The activity was lost after treatment with 2-mercaptoethanol.