The immunologic effects of a year-long therapeutic trial with the synthetic thymic hormone thymopentin were investigated in 29 patients with human immunodeficiency virus--induced lymphadenopathy syndrome. Peripheral T4 and T8 lymphocyte subsets and in vitro immunoglobulin synthesis with pokeweed mitogen (PWM) were monitored before treatment, every 4 months during treatment, and after treatment. Significant increases in circulating T4 cells and a reduction of PWM-induced immunoglobulin suppression were noted in the 21 patients who completed the trial. Furthermore, addition of thymopentin to the in vitro cultures improved the PWM response of T and B cells. After thymopentin treatment, a slight variability of clinical symptoms, including weight loss, diarrhea, night sweats, or a combination of these, was also noted in several subjects.