Sixteen patients with small cell carcinoma of the lung were immunocytochemically studied for CEA, keratin, SC and NSE. Seven of these patients were examined by electron microscopy to assess the multipotential in the differentiation of small cell carcinoma.NSE stain was positive in 10 of 16 patients; neurosecretory granule was positive in four of seven. These findings represent features of neuroendocrine differentiation of small cell carcinoma.In contrast, keratin was positively stained in six patients, filament in one, desmosome in five secretory granules in five and microvilli in one.These findings led us to conclude that small cell carcinoma may differentiate to epithelium in cellular characteristics.