CD3 antigen, formerly thought to be specific for T lymphocytes, has been identified in Purkinje cells of the cerebellum and gastric parietal cells in several species, including humans. The antibodies commonly used to recognize CD3 are directed against the epsilon-subunit of the T cell receptor. This subunit has a role in signal transduction in T lymphocytes and possibly other types of cells. We immunostained sections for CD3 from normal kidneys of several species, including humans, and from different primary human renal cortical neoplasms to determine if CD3 antigen is expressed in normal and in neoplastic tubular epithelium. CD3 expression was strong in normal proximal and distal tubular epithelium in most species and in renal oncocytomas, weak in chromophobe carcinoma, and negative in clear cell carcinomas, in papillary renal cell carcinoma, and in a transitional cell carcinoma. These findings suggest that this marker may be useful in the diagnostic workup and classification of renal cortical neoplasms.