A panel of hybridomas secreting monoclonal antibodies (mAb) reacting with antigens on mouse eosinophils and neutrophils has been selected. It is shown that four mAb bind preferentially to eosinophils, recognizing antigens expressed between 5 and 25 times more densely on these cells than on neutrophils. One mAb reacts preferentially with neutrophils, binding an antigen about 12 times more common on these cells than on eosinophils. Four mAb are shown to react with both eosinophils and neutrophils, and also macrophages. These mAb confirm the presence of different differentiation antigens on both eosinophils and neutrophils, and show that the myeloid series express antigens not expressed on lymphocytes.