Alpha B‐crystallin is a lens protein showing homology with small heat‐shock proteins. We have previously demonstrated its expression in non‐lenticular normal and diseased human tissues by immunostaining with a polyclonal antibody. In view of the expression seen in normal renal tubular epithelium, we have assessed the immunoreactivity of a variety of epithelial tumours, to determine the usefulness of alpha B‐crystallin as a specific renal tumour marker. Carcinomas arising from tissues which normally express alpha B‐crystallin, such as colo‐rectal and thyroid carcinomas, showed a varying pattern and degree of immunoreactivity. The most consistently positive tumours, however, with typically strong cytoplasmic and cell membrane staining, were renal cell carcinomas, 90 per cent of which showed positive immunoreactivity. This pattern of staining, while not absolutely specific, is a useful aid to the diagnosis of renal carcinoma. When a metastic deposit or a small biopsy is being assessed, anti‐alpha B‐crystallin may be included in a panel of antibodies, the pattern of staining of which may direct the search for the primary site of the tumour.