Quantitative and qualitative changes in cellular actin were followed during differentiation of a myeloid leukemia cell line, namely Ml, which was inducible with conditioned medium (CM) . During 3 d of incubation with CM, when the Ml cells differentiated to macrophages and lost their mitotic activity, the actin content, F-actin ratio in total actin, and the actin synthesis showed an increase . A greater difference before and after differentiation was found in the ability of G-actin to polymerize . Actin harvested from CM-treated cells showed a greater ability to polymerize, depending on the increased concentration of MgC12 and/or KCI and proteins, as compared with the actin from untreated Ml cells . Actin harvested from the Mml cell line, a macrophage line, had a particularly high polymerizability with or without CM treatment. In contrast, the actin from the D -subline, which is insensitive to CM, showed almost no polymerization .In recent years, the molecular mechanism of cell motility and phagocytosis has been studied with special attention given to the possible involvement of actomyosin in these cellular functions (7,15,23,36,38) . In nonmuscle cells, actin is not always in a filamentous or organized state as seen in muscle cells, but there is a time-to-time conversion between monomeric and filamentous states under a certain control mechanism, depending on the demand from cellular functions; resting, dividing, moving, and phagocytizing. Those nonmuscle cells contain a higher concentration of actins than the critical concentration required for polymerization, but most appear to be prevented from polymerization (4, 42) . Profilin, one of the inhibiting factors of actin polymerization, has been isolated from calf spleen (6) and human platelets (31) . On the other hand, Bray and Thomas (4), Abramowitz et J . CELL BIOLOGY