When human myeloid and monocytoid leukemic cell lines HL-60 and U937, respectively, were treated with an exogenous sialoglycosphingolipid, ganglioside GM3, in serum-free medium, cell growth was markedly inhibited, and their morphological maturation along a monocytic lineage was observed. In addition to a significant increase in phagocytic and nonspecific esterase activities, marked increase of monocyte-specific surface antigens detectable with monoclonal antibodies such as OKM1 and OKM5 was observed in GM3-fed cells. Other sialoglycosphingolipids with the carbohydrate structure belonging to ganglio-series oligosaccharide, ganglioside GM1 and a brain ganglioside mixture, had no effect on the cell differentiation, showing instead stimulatory actions on the growth of these cell lines. We recently demonstrated that the ganglio-series ganglioside GM3 characteristically increased during macrophage-like cell differentiation of these cell lines. The present results indicate that ganglioside molecular species that specifically increase during monocytic cell differentiation of human myeloid and monocytoid leukemic cell lines may play, in turn, an important role in the differentiation-induction of these cell lines along a monocytic cell lineage.