Differentiation-inducing agents have recently been applied clinically in various myeloproliferative diseases, based on their in vitro ability to induce differentiation in myeloid and monocytic cell lines. In this study we compared the abilities of two agents, dibutyryl cyclic AMP (DBcAMP) and retinoic acid (RA) to induce monocytic functions in the human histiocytic lymphoma cell line U937. Both agents produced similar induction of phagocytosis and reduction of nitroblue tetrazolium (NBT). Other monocytic functions, including accumulation of lysozyme, spontaneous and directed migration toward zymosan-activated serum (ZAS), the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) and leukotriene B4 (LTB4) were induced by DBcAMP, while RA induced migration toward LTB4 only. Simultaneous treatment with both inducers proved synergistic only with respect to phagocytosis. NBT reduction and migration toward FMLP and LTB4 were unchanged, while migration toward ZAS and lysozyme accumulation were suppressed by the addition of RA. The results suggest that the inducer affects the expression of cellular functions and characteristics specific to a particular lineage. In addition, the data presented indicate that the U937 cell line may serve as an excellent model system for the study of the regulation and mechanisms of chemotactic receptor expression in monocytes.