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β-Endorphinlike immunoreactivity and somatostatinlike immunoreactivity were detected in the mucosa and muscle layer of normal gastric antrum and corpus and in moderately differentiated adenocarcinoma derived from the antral mucosa. The concentration of β-endorphinlike immunoreactivity in the normal gastric tissues was 4–15 pmol/g wet wt tissue; this value varied from 9.64 to 241.39 pmol/g wet wt tissue (81.38 ± 37.82 pmol/g wet wt tissue) in adenocarcinomas. The concentration of somatostatinlike immunoreactivity was 18–25 pmol/g wet wt tissue in normal gastric mucosa, whereas it was 1–2 pmol/g wet wt tissue in adenocarcinomas. Gel exclusion chromatography of β-endorphinlike immunoreactivity revealed two peaks corresponding to β-endorphin and β-lipotropin. In normal mucosa and in the whole layer of antrum, the major peak was eluted near the position of β-lipotropin, and the minor broad peak was eluted near the position of β-endorphin. In contrast, in adenocarcinoma, β-endorphinlike immunoreactivity was eluted broadly at the position of β-endorphin and the other smaller peak was at the position of β-lipotropin. Gel exclusion chromatography of somatostatinlike immunoreactivity also showed different patterns between antral mucosa and adenocarcinoma. This study revealed the presence of the opioid peptide, β-endorphinlike immunoreactivity, not only in normal gastric tissue but also in adenocarcinomas with highly increased concentration and different elution patterns in combination with decreased concentration of somatostatinlike immunoreactivity.