We investigated the biological activity of IL-4 to murine connective tissue-type mast cells (CTMC). When purified peritoneal mast cells, typical CTMC, were incubated with pokeweed mitogen-stimulated spleen cell-conditioned medium (PWM-SCM) in methylcellulose, about one-fifth of mast cells showed clonal growth. Recombinant IL-4 alone did not stimulate the clonal growth, and purified IL-3 alone induced development of a small number of tiny clusters. In contrast, addition of IL-4 to IL-3 increased the number of clusters by a factor of 10. The number and size of clusters induced by the combination of IL-3 and IL-4 were comparable to those of mast cell clusters induced by PWM-SCM. The present results indicate that IL-4 is an essential factor for in vitro clonal growth of CTMC.