Résumé : The relationships between some parameters of the immune response and selenium were investigated in five patients receiving home parenteral nutrition for short-bowel syndrome. They were first submitted to a relative depletion by providing 20 micrograms selenium/day as L-selenomethionine for 1 mo. Then, daily selenium intake was raised to 200 micrograms for 2-4 mo. On entering the study, the patients presented a relatively good health status, and immunological parameters were at the lowest limit of the normal range. Four patients rapidly responded to the 200-micrograms supplementation by a continuous increase in their plasma selenium levels, whereas the fifth patient showed a moderate and late increase. At the end of the trial, there was an improvement in the lymphocyte response to pokeweed and phytohemagglutinin mitogens in four patients and to CD3 in three patients. The response to two of three antigens (Candidin, Varidase) tested was also enhanced in the same patients, but the response to the third antigen (tetanus toxoid) was uniformly low in all patients. The only patient showing essentially no immune improvement after selenium supplementation was the one with a low and delayed increase in plasma selenium. This study supports a role for selenium in the maintenance of an optimal immune response in humans.