Immune complexes react with neutrophils and macrophages by way of surface membrane Fc receptors. Alternatively, after fixation of complement, interaction may be with membrane C3b receptors. As a result, phagocytosis is induced. A consequence of this process is the liberation to the extracellular medium of lysosomal constituents. In neutrophils, fusion of granules with phagocytic vacuoles liberates the contents to the outside because some of the vacuoles, for various reasons, are inefficiently closed off. Alternatively, reaction of neutrophils in vitro or in vivo with immune complexes and/or activated complement components (C3b, C3a, C5a, C567) on surfaces too large to be phagocytosed, induces direct exocytosis of granules with consequent release of constituents. In both cases the liberation of lysosomal constituents is a secretory process, does not involve cell lysis, and resembles closely the secretory reactions of other widely differing cell types.