Malignant infantile osteopetrosis is a severe congenital disease characterized by impaired osteoclast activity. Among the multiple factors that influence bone resorption, the c-src protooncogene product pp60c-src plays an essential role, since mice which lack pp60c-src develop osteopetrosis. To gain insight into the possible role of pp60c-csrc in the pathogenesis of infantile osteopetrosis, we examined the osteoclasts of three children displaying the typical features of the disease, aged respectively one, four and seven months. pp60c-csrc expression and localization, together with the expression of a 80/85-kilodalton pp60c-src substrate, cortactin, were examined by immunoelectron microscopy. Osteoclasts from two giant cell tumors were used as controls. Bone and tumor samples were fixed in 4% paraformaldehyde, included in LR-White resin at -30 degrees C and the sections processed with mAb 327 or mAb anti p80/85 by an immunogold technique. pp60c-src was expressed in the cytoplasm, in nuclear membranes and in nuclei of the osteoclasts of the three osteopetrotic children. The subcellular localization of the kinase was not different from the localization in giant tumor cells. In both cases cortactin was abundant. In conclusion, in three children with malignant osteopetrosis, pp60c-src expression in osteoclasts does not appear to be involved in the pathogenesis of the disease. The presence of this protein, however, does not necessarily reflect normal c-src tyrosine kinase activity, nor normal c-src-dependent intracellular signaling pathways. Moreover the presence of the protein in nuclear membranes, and especially around nuclear pores supports the hypothesis that in osteoclasts, c-src may participate in the regulation of RNA processing.