The Wilms' tumour gene (WT1) product is expressed during the development of the urogenital system. This study was undertaken to evaluate four anti-WT1 antibodies and use the most specific one to examine the expression of WT1 in formalin fixed, paraffin wax embedded tissues from human embryos, fetuses, and paediatric renal neoplasms. The antibodies were assessed on paraffin sections of fetal kidney and by western blotting. Immunohistochemical techniques were optimised and performed on a range of embryonic, fetal, and infant tissues from 35 days post-conception to three months of age, and on a selection of paediatric renal neoplasms. The antibodies tested were found to vary in their specificity. Anomalous expression in smooth muscle was seen with one batch of a commercial polyclonal antibody. WT1 protein was detected in both the metanephros and the mesonephros, the spleen, the gonads, and in the peritoneal mesothelium in fetuses. WT1 was expressed in nuclei and was strongest in the podocytes of fetal kidney. The podocytes of infant glomeruli were also positive. There was focal positive staining in Wilms' tumours, nephrogenic rests, and in a cystic partially differentiated nephroblastoma. Staining of nuclei was seen in one of two rhabdoid tumours of the kidney. No positive staining was seen in other renal tumours. WT1 is detected readily in formalin fixed material. There were differences in specificity between batches of the polyclonal antibodies used. The distribution of the WT1 gene product in tissues and tumours reflected previous findings with in situ hybridisation studies of WT1 mRNA.