The monoclonal antibody PAL-E is specific for endothelial cells in a wide variety of normal and tumour tissue. In normal kidney, PAL-E reacts exclusively with the endothelium of non-glomerular blood vessels. In Wilms' tumour, binding of PAL-E was not restricted to the endothelium; staining of blastemal cells was observed in seven out of eight cases examined. Mesenchymal and tubular components, if present in Wilms' tumour, were negative. In contrast, a monoclonal antibody to Factor VIII-related antigen (RFF-8-R-1) bound only to endothelial cells in these tumours. In fetal kidney, PAL-E binding showed a wider distribution than in adult kidney and both stromal and glomerular capillaries were stained. Tubules and non-endothelial stromal cells were negative. These results indicate that the reactivity of the monoclonal antibody, PAL-E, is not restricted to cells of endothelial origin in all tissues. The implications of these findings for the differentiation of Wilms' tumour are discussed.