This combined ultrastructural and immunohistochemical study of four alveolar soft-part sarcomas analyzes the characteristic granules in order to elucidate the histogenesis of this unusual neoplasm. The granules have the ultrastructural features and staining characteristics of renin, react with antiserum to renin, and like renin, incorporate zinc into the crystalline storage granules. The cell appears to be a modified smooth muscle cell analogous to renal juxtaglomerular cells and the cells of juxtaglomerular cell tumors. As there is no association with hypertension, it would appear that alveolar soft-part sarcomas either do not secrete or the product is a form of renin that does not act upon the angiotensin system.