Rabbit neutrophil leukocytes incubated with immune precipitates, with rabbit γ-globulin heat-aggregated at 68 °C, or with rabbit γ-globulin-coated latex particles, release lysosomal constituents such as basic proteins, β-glucuronidase and vascular permeability inducing substances. The release is not accompanied by cytolysis. Uncoated latex particles or γ-globulin heated to 75 °C are taken up by the neutrophils, but this uptake is not associated with release of lysosomal constituents. The release of lysosomal constituents does not parallel the degree of phagocytosis, suggesting that this release is not attributable entirely to phagocytosis. It is postulated that the release is triggered by the reaction of neutrophils with immunoglobulin which has undergone configurational changes, by interacting with antigen, by heat or by adsorption to latex particles.