Effect of methisazone and other drugs on mouse hemophoietic colony formation in vitro.

Methisazone (N-methylisatin beta-thiosemicarbazone) was shown to inhibit the development of hemopoietic colonies in vitro when incorporated in the culture medium. The activity of methisazone was compared with that of several antimetabolic and antibiotic drugs. Although methisazone reduced the number of colonies, it did not affect the size of those colonies which did develop, and evidence is presented which shows that it acted only at the early stages of colony development. Some implications of these findings are briefly discussed.