RIA method was used in this study to determine immunoreactive beta-endorphin (ir-beta-EP) in gastric mucosa of patients with benign and malignant gastric diseases. The results showed that the content of ir-beta-EP in gastric mucosa in the peptic ulcer group was the highest (68 +/- 9.5 pg/mg wet weight tissue, P less than 0.01), while its content in gastric carcinoma was closely related to the degree of differentiation of the tumor, that is, in poorly differentiated carcinoma it was lower than that in well differentiated carcinoma (P less than 0.02), and was also lower than that in gastritis (P less than 0.05). At the same time, we found that beta-endorphin can markedly augment the 3H-TdR incorporation of lymphocytes (P less than 0.01). This effect was not blocked by naloxone.