F-actin filaments are disrupted by the action of cytochalasin and glutaraldehyde. Muscle tropomyosin which is able to polymerize can protect F-actin against fragmentation caused by these two agents. This protective effect does not occur with nonpolymerizable, brain or carboxypeptidase A-treated skeletal muscle tropomyosins. The protection of F-actin against the action of cytochalasin and glutaraldehyde takes place under conditions where the F-actin filaments are saturated with tropomyosin, that is, at a molar ratio of tropomyosin to actin of 1 : 7. It is suggested that nonpolymerizable tropomyosin lacks the protective ability because its binding to F-actin is considerably weaker than the polymerizable tropomyosin and does not saturate all of the binding sites on F-actin.