Cloning and nucleotide sequence analysis of the human calcitonin mRNA from the BEN lung carcinoma cell line, a cell line known to secrete high-Mr forms of calcitonin, showed no difference in the coding region at the nucleotide level compared with calcitonin mRNA isolated from medullary thyroid carcinoma which secretes calcitonin monomer. Therefore, the secretion of high-Mr forms of calcitonin reflects the absence or limited activity of proteolytic processing enzymes within the secretory pathway of this cell line. In all other respects, as judged by RNA blotting and S1 mapping, calcitonin/alpha-CGRP expression was identical to that found in medullary thyroid carcinoma, including the differential use of an alternative splice donor site within intron 1. The BEN cell line also produces low levels of alpha-CGRP mRNA and secretes CGRP antigenic determinants. Analysis of plasma CGRP levels in 12 patients with anaplastic lung carcinoma showed elevated levels in 11 of these, suggesting that CGRP may be an important diagnostic marker for this disease.