Proliferation of murine mast cells is induced by both T-cell-derived and fibroblast-derived growth factors. Because the most potent T-cell- derived mast cell growth factor, interleukin-3, promotes the migration of mast cells, we investigated whether fibroblast-derived growth factors had the chemoattractive activity as well. Conditioned medium (CM) of BALB/3T3 fibroblasts induced the migration of cultured mast cells (CMC) derived from normal (+/+) mice. BALB/3T3-CM contained the mast cell growth factor (MGF)/stem cell factor (SCF)/kit ligand (KL), which is the ligand for the receptor encoded by the W (c-kit) gene. CMC derived from the spleen of W/W mice lack the extracellular domain of the W (c-kit) receptor, and W/W CMC did not proliferate in response to BALB/3T3-CM. However, W/W CMC did migrate normally toward BALB/3T3-CM and, moreover, the antibody to the extracellular domain of the W (c- kit) receptor did not inhibit the chemoattractive activity of +/+ CMC toward BALB/3T3-CM. These results indicated that MGF/SCF/KL itself did not represent the major chemoattractive activity. On the other hand, BALB/3T3-CM induced neither proliferation nor migration of CMC derived from mi/mi mice. Both W/W and mi/mi mice are deficient in mast cells, but the present results suggest that the mechanism of the abnormality is different between W/W and mi/mi mice.