Latrunculin A, a toxin from a Red Sea sponge, was shown to be a very potent inhibitor of immunological phagocytosis by normal and activated macrophages (obtained from mice injected i.p. with LPS), as well as by polymorphonuclear leukocytes. This toxin blocks the interiorization of the immune complexes but does not interfere with their binding to the phagocyte (recognition phase); activated macrophages were more susceptible to this inhibition than normal macrophages and polymorphonuclear leukocytes. The effect of the toxin on cellular spreading of macrophages was also studied using two kinds of substrate: glass, and glass covered with IgG immune complexes. Latrunculin A was able to impair the spreading of normal macrophages on glass covered with immune complexes, and could also completely reverse the spreading after it had occurred. Contrarily, in activated macrophages, this toxin could neither impede the spreading nor reverse it, a difference that might be a distinctive property of the activated state. We have also found that latrunculin A can reduce the percentage of F-actin in both normal and activated macrophages, the activated cells being more susceptible to this effect. Since latrunculin A is a blocking agent of actin polymerization in vitro, these results indicate that actin polymerization and assembly must be an essential component of the primary, active event of the engulfment phase of phagocytosis.