We investigated the effect of phytohemagglutinin and the neuropeptide beta-endorphin on the in vitro lymphocyte proliferative response in patients with chronic ulcer disease and upper gastrointestinal bleeding or in remission, acute ulcer or hemorrhagic gastritis, and in healthy persons without upper gastrointestinal disease. We found that the addition of beta-endorphin produced a strongly enhanced proliferative response to phytohemagglutinin in lymphocytes from patients with chronic ulcer disease, and in particular from those with recent bleeding.