Several monoclonal antibodies against human leukocyte differentiation antigens have been shown to react with normal kidney. Four monoclonal antibodies with different patterns of reactivity on normal kidney were tested against 20 renal epithelial neoplasms and 5 Wilms' tumors. In general, the expression of these antigens on renal tumors was faithful to their presence on normal kidney; this finding suggested that most renal tumors may be derived from proximal tubule epithelium. There was, however, both intertumor and intratumor heterogeneity of expression of these antigens, but the different phenotypes encountered did not correlate in any simple way with histologic subclassification. In contrast, the five Wilms' tumors were phenotypically different from the epithelial neoplasms, consistent with an origin from a more primitive renal cell.