We used antibodies to human leukocyte ("neutrophil") elastase and cathepsin G to localize the corresponding antigens in human neutrophils, monocytes, and alveolar macrophages by immunohistochemistry. Furthermore, we combined immunogold localization with enzyme histochemistry to localize proteinase antigens and endogenous peroxidase activity in the same sections. As expected, all neutrophils contained both elastase and cathepsin G, and the proteinases localized to granules with peroxidase activity. In contrast, marked heterogeneity in monocyte staining for elastase, cathepsin G, and endogenous peroxidase was found. Sixty percent or more were unstained, while the remainder varied greatly in staining intensity. The elastase and cathepsin G in monocytes were localized by immunoelectron microscopy, combined with histochemistry, to cytoplasmic granules which had peroxidase activity. Alveolar macrophages were unstained. Therefore, a subpopulation of peripheral blood monocytes contains leukocyte elastase and cathepsin G in a cell compartment from which these enzymes may potentially be released into the extracellular space. The occurrence of peroxidase and neutral proteinases in the same granules in monocytes could permit the H2O2-myeloperoxidase-halide system and the neutral proteinases to act in concert in such functions as microbe killing and extracellular proteolysis.