Continuous cell lines of human lung cancers were analyzed for neuron specific enolase (NSE). The levels of NSE in the small cell carcinoma cells were in every case much higher than those derived from other forms of lung cancer. Since NSE is strictly localized to neurons and neuroendocrine cells of the amine precursor uptake and decarboxylation (APUD) series, the data provided further evidence that small cell carcinoma is an APUD cell tumor and is distinct from other forms of lung cancer.