A highly sensitive and specific radioimmunoassay for human calcitonin gene-related peptide (CGRP) has been developed and used for determination of serum CGRP levels in 232 normal individuals, 123 females and 109 males, range of age 18 to 79 years. Mean serum concentration was 36.3 pmol/l +/- 6.2 (SD), and serum levels were unrelated to age and sex. The specificity of the assay was confirmed by gel chromatography, followed by HPLC analysis of extracts from normal serum using synthetic alpha- and beta-CGRP as standards.