Immunohistochemistry was applied to identify the nature of the nucleated cells that accumulate in the vasa rectae of the corticomedullary junction in acute tubular necrosis. In all 6 cases studied, there were intravascular cells that reacted with monoclonal antibodies to erythroblast, macrophages, myeloid cells, T and B lymphocytes and rare megakaryocytes. The findings are consistent with the occurrence of intravascular haematopoiesis in the renal medulia in acute tubular necrosis.