Immunoreactive gastrin-releasing peptide (IR-GRP) was found to be present in medullary carcinoma of the thyroid (MCT) by use of a radioimmunoassay (RIA) specific for the carboxyl-terminal portion of GRP. Immunohistochemical studies revealed IR-GRP in the MCT tumor cells, indicating that the tumor cells produce IR-GRP. Immunoreactive GRP was also detected in macroscopically normal thyroid tissue of patients with multiple endocrine neoplasia type II (MEN II, or Sipple's syndrome) and in areas of C cell hyperplasia and micronodules in the thyroids of patients with MCT. When these tissue extracts were examined with a bombesin RIA that recognizes bombesin but not GRP, no IR-bombesin was detected, suggesting that the IR-GRP detected in these tissues is more similar to GRP than to bombesin. IR-GRP was also undetectable in normal thyroid tissues. Plasma IR-GRP was also undetectable in normal thyroid tissues. Plasma IR-GRP was elevated to 130 to 780 pg/mL (normal: undetectable, less than 62.5 pg/mL) in three patients with metastatic MCT, and both calcium and tetragastrin increased the plasma levels of IR-GRP. Sephadex G-50 gel filtration of the MCT extracts revealed two peaks, one coeluted with porcine GRP (1-27) and the other eluted just after its carboxyl-terminal (14-27) fragment. There was a significant correlation (P less than 0.01) between the concentration of IR-GRP and that of IR-calcitonin in MCT tumor tissue and in macroscopically normal portions of thyroid tissue from two patients with MEN II, although the concentration of IR-GRP was only about 0.1% of that of IR-calcitonin.(ABSTRACT TRUNCATED AT 250 WORDS)