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 MgCl2 and/or KCl 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.

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