AbstractNeutrophil‐induced alterations in feline erythrocytes were studied to better understand the pathogenesis of erythrocyte destruction associated with inflammatory diseases. As in previous studies, addition of superoxide dismutase/catalase to a coculture of erythrocytes and activated neutrophils attenuated neutrophil‐induced immunoglobulin G (IgG) binding. However, incubation of erythrocytes with hydrogen peroxide or neutrophil‐derived anuclear cytoplasts (neutroplasts) failed to induce IgG binding. Addition of phenylmethylsulfonyl fluoride, a serine protease inhibitor, to the erythrocyte‐neutrophil coculture attenuated IgG binding. These observations suggest that neutrophil‐derived serine protease activity is involved in IgG binding to erythrocytes. Further, incubation of erythrocytes with serine proteases, but not metalloproteases or sulfhydryl proteases, induced immunoglobulin binding. Freeze‐fracture replicas of the erythrocyte membrane failed to demonstrate clustering of band 3 protein, suggesting that spatial rearrangement of band 3 protein was not the cause of the IgG binding. Neutrophil‐induced IgG binding due to the combined action of proteases and oxidants may explain the accelerated destruction of erythrocytes in inflammatory diseases.