Retinoic acids (RAs) exert pleiotropic effects on cellular growth and differentiation. All-trans retinoic acid (ATRA) and 9-cis retinoic acid (9-cis RA), a stereoisomer of ATRA, induce differentiation of leukemic cell lines and cells from patients with acute myelogenous leukemia (AML) in vitro. Despite information on the effects of RAs on hematopoietic cells, little is known about how RAs act on the hematopoietic microenvironment, especially on bone marrow stromal cells. Based on recent observations that various cytokines produced mainly by bone marrow stromal cells regulate hematopoiesis, we analyzed the effects of RAs on cytokine production by these cells. ATRA or 9-cis RA treatment of human bone marrow stromal cell line KM101, which produces macrophage colony-stimulating factor (M-CSF) and granulocyte- macrophage colony-stimulating factor (GM-CSF) constitutively, enhanced mRNA levels of both cytokines in a dose-dependent manner. Both RAs also stimulated M-CSF production from primary cultures of human bone marrow stromal cells. Both retinoic acid receptor (RAR)-alpha and retinoid X receptor (RXR)-alpha were expressed constitutively in KM101 cells. ATRA did not affect the expression of either receptor, whereas 9-cis RA increased RXR-alpha mRNA expression in a dose-dependent manner, but did not affect levels of RAR-alpha mRNA. These findings may have important biologic implications for both the role of RAs in hematopoiesis and the therapeutic effects of ATRA on the hematopoietic microenvironment in patients with acute promyelocytic leukemia (APL).