Recently, a novel cytokine designated stem cell factor (SCF) was isolated from medium conditioned by buffalo rat liver cells and proved to be the ligand for c-kit. We have examined the effects of recombinant rat SCF alone and in various combinations with interleukin-3 and interleukin-4 on murine mast cell colony formation in methylcellulose culture. As a source of connective tissue-type mast cells (CTMC), we used peritoneal mast cells. No individual factor supported colony formation by purified peritoneal mast cells. When cells were grown in combinations of two factors, significant mast cell colony growth was seen. When cells were grown in the presence of three factors, not only the number of colonies was increased but also the colonies were larger. Mast cells in these colonies contained safranin- and berberine sulfate-positive cells, but the proportions of positive and negative cells varied depending on the factor combinations. We then examined the effects of these factors on proliferation of bone marrow-derived mast cells (BMMC) by replating pooled mast cell colonies. As a single factor, only interleukin-3 supported mast cell colony formation. Combinations of two of the three factors supported mast cell colony formation. However, the most impressive synergism was seen again with the combination of the three factors. Not only was the number of colonies increased, but there was a significant increase in size. These results indicate that SCF is an important factor for the proliferation of both CTMC and BMMC.