Neutrophil-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.