Adult and fetal human kidneys were investigated for the reactivities of monoclonal antibodies, BA-1, BA-2 and BA-3 against human leukocytes. In developing metanephros, their reactivities changed reflecting the stage of nephrogenesis. Thus BA-1 stained both metanephric blastema and ureteric bud. Glomerular and proximal tubular development was characterized by the disappearance of BA-1 reactive antigen and the appearance of CALLA defined by BA-3. Immuno-electron-microscopically CALLA was solely located on the glomerular epithelial membrane and on the microvilli of the proximal tubules. BA-2 constantly stained ureteric bud-derived tissues. These observations were applied for the analysis of histogenesis of Wilms' tumour. Tumour blastema of the classical type reacted only with BA-1. Epithelial components in the classical as well as the epithelial type reacted both with BA-1 and BA-2. CALLA was only detectable in glomeruloid and the connecting tubular structures, while these were unstained by BA-1. Stromal components in the classical and the sarcomatous type did not express any of these antigens. Staining patterns were identical in surgically removed and xenotransplanted tumours. These studies establish that each component of Wilms' tumour can clearly be interpreted regarding its histogenesis and that epithelial components corresponding to proximal tubules are identified in Wilms' tumour.