The retinoic acid-stimulated mouse-transformed epidermal cell line Pam 212 generated soluble mediators (RA-Pam sup) which showed mast cell-proliferating activity and induced mast cell-like colonies from bone marrow cells. Both mucosal-type and connective tissue-type mast cells were demonstrated on histochemical analysis. The 3T3 fibroblast cell line or human trichilemmomal cell line (TL1) also generated similar molecules on retinoic acid stimulation. Anti-stem cell factor antibody presented a rather more potent inhibitory activity on RA-Pam sup-dependent mast cell colony formation than did anti-IL3 and -IL4 antibodies. RA-Pam sup only induced mast cell colonies in semisolid culture and failed to promote mast cell growth in a suspension culture. In addition, mast cell colonies showed close contact with fibroblast-like cells on the methylcellulose plate, but this finding was not observed in culture without RA-Pam sup.