Plasminogen activator (PA) was located in newborn rat osteoclasts using a single-cell assay. Immunohistochemistry using biotin-streptavidin-peroxidase indicated the presence of both tissue-type plasminogen activator (tPA) and urokinase (uPA) within the cytoplasm of osteoclasts isolated from newborn rat long bones. Electron microscopic immunohistochemistry using the biotin-streptavidin-colloidal gold system on L.R. Gold thin resin sections of undecalcified, newborn rat tibial metaphyseal trabecular bone identified these proteases in the lysosomal network of osteoclasts. uPA was also localized in marrow macrophage lysosomes, but tPA was not detected in these cells. The localization of these enzymes within osteoclasts may imply their involvement in bone resorption.