Low concentrations (greater than or equal to 10(-7) M) of cytochalasin B reversibly inhibit the temperature-dependent gelation of actin by an actin-binding protein. The cytochalasin B concentrations which maximally inhibit actin gel formation are 10-fold lower than the concentrations which maximally impair phagocytosis by intact macrophages. Cytochalasin B also prevents the polymerization of monomeric actin in sucrose extracts of macrophages in the absence but not the presence of 0.1 M CKl. 10(-6) M cytochalasin B dissolves macrophage extract gels and gels comprised of purified actin and actin-binding protein by dissociating actin-binding protein from actin filaments. This concentration of cytochalasin B, however, does not depolymerize the actin filatments.

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