An antibody to pure human renin and an immunoperoxidase technique were used to stain immunoreactive renin in nephroblastomas. In an unselected series of surgically removed nephroblastomas immunoreactive renin was found in 10 of 19 tumours (53%). Tissue obtained at necropsy from 12 cases of disseminated nephroblastoma was also studied, and immunoreactive renin in metastatic tumours was found in three cases. Patients' blood pressures and serum potassium concentrations showed no evidence of excess renin activity. It is therefore suggested that if nephroblastomas contain renin as commonly as this study suggests, then any hormone secreted may be biologically inactive.