Mouse monocytic Mm-A cells are a highly leukemogenic variant line of the monocytic and non-leukemogenic cell line Mm-1, which developed spontaneously from mouse myeloid leukemia M1 cells. Studies were made on whether Mm-A cells could be induced to differentiate further by agents that were effective for inducing differentiation of the parent M1 cells and other leukemic cells. Of the agents tested, butyrate, conditioned medium from concanavalin A-stimulated spleen cells, lipopolysaccharide (LPS) and N6,O2-dibutyryl adenosine 3'5'-cyclic-monophosphate (dbcAMP) significantly stimulated the lysozyme activity of Mm-A cells, which is one of the most characteristic biochemical markers of monocytes and macrophages. Butyrate was the most effective agent for increasing lysozyme production by Mm-A cells; culture with 0.5mM butyrate for 3 days increased lysozyme production by Mm-A cells about 50-fold. Inducers of M1 cell differentiation such as dexamethasone, 1 alpha,25-dihydroxyvitamin D3, arginase, and proteinous inducer did not increase the lysozyme activity. Butyrate also induced NBT reduction and stimulated other differentiation-associated functions, such as expressions of Fc receptors on the cell surface, immune phagocytosis and production of inducer for M1 cell differentiation. Its effect in stimulating differentiation of Mm-A cells was synergistic with that of dbcAMP or LPS. Incubation with butyrate inhibited the proliferation of Mm-A cells, about 0.3mM butyrate causing 50% inhibition. These results indicate that monocytic, leukemogenic Mm-A cells can be induced to differentiate further by butyrate and that the inducers of differentiation of Mm-A cells are markedly different from those of the parent myeloblastic M1 cells.