Fourteen primary and nine metastatic renal cell carcinomas were examined with a commercially available antibody against prealbumin (transthyretin) using the avidin-biotin-peroxidase complex (ABC) technique. Twenty-two of the 23 tumours expressed prealbumin, and in 16 of them more than 50% of the tumour cells showed immunoreactivity. The same tumours were subjected to immunostaining with antibodies against low molecular weight keratin, carcino-embryonic antigen, S-100 protein and epithelial membrane antigen for comparison. We conclude that prealbumin, which was absent in 14 adenocarcinomas from other organs, should be included in the panel of markers for differentiating metastatic adenocarcinoma of renal origin from those of other primary tumours.