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Mouse macrophages, activated in vivo or in vitro, were made to internalize sheep erythrocytes opsonized with IgG or IgM and complement. Scanning electron microscopy and transmission electron microscopy were performed ac various times after the transfer of the cultures from conditions favoring attachment to conditions favoring internalization. The receptors for Fc and C3 were distributed randomly over the macrophage surface with the exception of the extreme periphery, were Fc receptors were more abundant. The E-IgG were ingested by means of thin membrane extensions rising from the macrophage surface and enclosing the opsonized particles tightly in a cup-like structure protruding from the macrophage surface. Only afterwards were the covered particles drawn into the cell body proper. The E-IgMC were seen to sink directly into the macrophage cytoplasm without apparent involvement of membrane extensions. Experiments with cytochalasin B suggested that microfilaments were essential for the phagocytosis by Fc but much less important with the C3 receptor.