Nineteen cytochalasins including 6 natural cytochalasins, 2 natural aspochalasins, 7 natural chaetoglobosins and 4 their acetates were investigated by viscometry to examine their effects on in vitro polymerization and depolymerization of microtubules. Cytochalasin A was proved as an only active cytochalasin, confirming the studies of Himes et al., and others showed very weak or no noticeable effects. This results indicate that a wide range of effects of cytochalasins are not related to interactions with microtubules.