The presence of mitogen(s) in leiomyoma extracts stimulating cells with the fibroblast, myoblast, and osteoblast phenotype was documented. Mitogenic activity in leiomyoma extracts was acid stable and sensitive to tryptic digestion. Reverse-phase high-performance liquid chromatography successfully separated mitogen(s) with preferential activity for myoblast cells from mitogens with a broad type of cell specificity and from inhibitors. This leiomyoma-derived preferential activity for myoblasts was absent in identically treated myometrial and endometrial extracts. This suggests that leiomyoma-derived substances with preferential growth factor activity for myoblast-like cells may play a role in the pathophysiologic characteristics of uterine leiomyomas.