Levels of immunoreactive (IR) human NH2 terminal (HNT) of pro-opiomelanocortin were determined serially in 80 patients with various pulmonary carcinomas, 20 patients with various pulmonary diseases without proven carcinoma, and 32 normal control subjects. Sixty +/- 14% (S.E.) (95% confidence interval) of the patients with proven pulmonary carcinoma had elevated values of IR HNT in plasma (greater than 100 pg/ml); the highest frequency of elevated IR HNT was found in patients with small-cell carcinoma [74 +/- 14.5% (95% confidence interval)]. In eight patients with pulmonary tumor who underwent surgical thoracic exploration, the plasma level of IR HNT in the pulmonary vein was 50.6 +/- 21.4% (95% confidence interval; N = 8) higher than that in the pulmonary artery. The levels of IR HNT correlated well with those of plasma adrenocorticotropic hormone (IR ACTH) (r = 0.988; p less than 0.001); however, the level of IR HNT was 3- to 4-fold higher than that of IR ACTH. Gel permeation chromatography of plasma of patients with pulmonary carcinoma revealed a major peak of IR HNT coinciding with the elution volumes of authentic HNT (Mr 12,000) and a minor peak of IR HNT near the void volumes. These results suggest that HNT originating from pulmonary carcinoma is chemically and immunologically similar to that from human pituitary gland and that measurement of plasma IR HNT may indicate the state of secretion and/or metabolism of peptides related to the expression of the pro-opiomelanocortin gene in human pulmonary carcinoma. The HNT immunoassay is a simple procedure which, contrary to that of ACTH, does not require plasma extraction prior to the assay. This suggests the possibility that measurement of IR HNT alone and in conjunction with other biochemical markers could be a useful parameter for diagnosis and follow-up of patients with pulmonary carcinoma.