Phagocytosis is a prime example of a cellular event in which cell surface perturbation activates the assembly of a filamentous gel beneath the plasma membrane. This gel may be responsible for movement of the membrane around ingestible particles. The molecular mechanism of these events is being approached by the purification of actin, myosin and associated proteins from phagocytic cells and by the study of a human disease, neutrophil actin dysfunction. Novel contractile proteins discovered in mammalian phagocytes include a cofactor that regulates actin:myosin interaction and an actin-binding protein that promotes assembly and gelation of actin. There is evidence that phagocytosis alters the state of the actin-binding protein, and that this alteration may be an early event in the assembly of the actin gel. Cytochalasin B, which inhibits phagocytosis, acts by interfering with the interaction between actin-binding protein and actin. Actin polymerized poorly in the neutrophils of a human infant, and the affected neutrophils were deficient in phagocytosis. Actin assembly is important in phagocytosis and is amenable to biochemical analysis.