Peptide alpha-amidation is a posttranslational modification of approximately half of all endocrine and neuroendocrine peptide hormones, including several hormones with mitogenic effects for tumor cells, and is typically essential for complete hormonal bioactivity. alpha-Amidated peptide hormones have been reported to be autocrine growth factors for small cell lung cancer cells. We report here that a variety of human lung tumor cell lines express both enzymes required for the two-step conversion of inactive glycine-extended peptides into their active COOH-terminal alpha-amide analogues. Human tumor cell peptidylglycine alpha-amidation enzymes are present in multiple molecular forms. Both proteins are metalloenzymes which are present at highest concentrations in secretory granules in neuroendocrine cell lines. The expression of these enzymes is positively correlated with expression of other markers of the neuroendocrine phenotype, such as DOPA decarboxylase. Peptidylglycine alpha-amidating enzyme-specific activities are approximately 50-fold higher in extracts of endocrine cell lines (lung small cell and carcinoid) than of nonendocrine lines. Biochemical characterization of the peptidylglycine alpha-amidating enzymes will enable development of tools for detection of endocrine processes in the early stages of neoplasia and for interruption of autocrine stimulation pathways in tumor cells.