We investigated the effect of a known T cell mitogen, phytohemagglutinin (PHA) and the neuropeptide beta-endorphin on the in vitro lymphocyte proliferative response in patients with ulcerative colitis and in health persons. We found that patients with ulcerative colitis show enhanced reactivity to PHA, when incubated in the presence of beta-endorphin. The activity of the disease during the period of the investigation seemed to play no role, since lymphocytes from both patients in exacerbation or remission reacted in a similar manner.