Human normal bone marrow cells were evaluated for alteration of differentiation after exposure for seven days to 10(-12)-10(-9) M cytosine arabinoside (ARA-C) in liquid culture. An increased number of induced cells had the morphologic appearance of mature monocytes-macrophages; they adhered to petri dishes, reacted positively to fluoride-sensitive naphthyl acetate esterase, and specifically bound My4 monoclonal antibody (MCAb). Assessment of phagocytosis and killing of Candida albicans (CA) by cultured monocytes-macrophages exposed to ARA-C demonstrated that treated cells had the same capacity to phagocytose and kill CA as did untreated cells. In semisolid culture, low doses of ARA-C did not affect myeloid colony growth. These studies indicate that ARA-C enhances monocytic differentiation of normal human bone marrow cells in liquid culture.