CD24 antigen is a glycoprotein expressed on haematopoietic cells, including B cells, T cells and granulocytes and on non-haematopoietic cells, including neural cells, ganglion cells and the cells of the adrenal medulla. The antigen is also expressed on renal cell carcinoma, small cell lung carcinoma and neuroblastoma. We have cloned rat cDNA encoding core polypeptide of CD24 antigen from embryonic brain and shown that the core molecule is highly expressed in embryonic brain and non-neural tissues. Rat tissue and various human neoplastic cell lines were investigated for the gene expression of CD24 core polypeptide by in situ hybridization. The transcript was localized in gastrointestinal epithelia, ductal and acinar epithelia of the salivary gland, the bronchiolar epithelium, renal tubular epithelium, the epithelium of the oviduct, follicular cells of the thyroid, medullary cells of the adrenal gland, Auerbach's plexus, B blastoid cells in lymph nodes, hair follicles, and the sweat glands of the skin. Among the various human neoplastic cell lines investigated, the transcript was detected in squamous cell carcinoma of the lung, gastric carcinoma, colon carcinoma, choriocarcinoma and renal cell carcinoma. The result suggest that the core molecule of CD24 antigen may be expressed in a wider range of epithelial cells and carcinoma cell lines than has been reported. Furthermore, we show that gene expression of CD24 core polypeptide is confined to the proliferative zone of the gastrointestinal mucosa, suggesting that core molecule is transiently expressed on the surface of epithelial cells in the process of cellular maturation. We discuss a possible role for CD24 antigen in the maturation of epithelial cells in the gastrointestinal tract.