Serum lysozyme activity and nitroblue tetrazolium (NBT) reduction by blood phagocytes were studied in 24 dogs with controlled insulin-dependent diabetes mellitus and in 16 healthy controls aged from 6 to 12 years. In dogs with diabetes mellitus serum lysozyme activity was significantly (p less than 0.001) lowered. The NBT reducing value of circulating phagocytes in diabetic dogs was increased (p less than 0.05), whereas these cells increase in NBT reduction following stimulation (stimulation index) was decreased (p less than 0.05). It is suggested that in diabetic dogs changes in activity of circulating and tissue phagocytes may occur.