Patients with severe thermal burns demonstrate a decreased cellular immunity. The purpose of this study was to ascertain whether thymosin enhances in vitro T-lymphocyte functions in such patients. Peripheral blood lymphocytes were obtained serially from 22 burned patients and 35 healthy adults. In vitro lymphocyte functions were evaluated by E-rosette formation, lymphocyte culture responses to PHA, Con A, PWM, PPD, SK-SD, mumps antigen, and tetanus toxoid, and mixed lymphocyte culture reactions. These tests were performed with and without in vitro addition of thymosin. Most of the parameters examined were significantly decreased in patients during the first 2 weeks postburn. The in vitro addition of thymosin significantly restored impaired lymphocyte responses, except in mixed lymphocyte culture reactions. These results demonstrate that thymosin enhances certain in vitro T-lymphocyte functions in burned patients. They further suggest that the administration of thymosin may restore decreased cell-mediated immunity in severely burned patients.