We have investigated the effects of combination therapy with thymosin alpha 1 and natural human lymphoblastoid interferon-alpha in human immunodeficiency virus infection and have shown that in vitro this combination treatment: (1) synergistically stimulated the cytotoxic activity against natural killer-sensitive target cells of lymphocytes collected from human immunodeficiency virus-infected donors and (2) did not interfere with the antiviral activity of zidovudine. We thus studied the effects of combination therapy with thymosin alpha 1, interferon-alpha and zidovudine in patients with CD4+ lymphocytes ranging from 200 to 500/mm3 in a randomized non-blinded study and found that the treatment was well tolerated after 12 months of therapy and was associated with a substantial increase in the number and function of CD4+ T cells. A similar effect was not observed in human immunodeficiency virus patients treated with zidovudine alone or associated with single agents. These data suggest the need for a controlled, double-blind clinical trial, recently initiated with the approval and the support of the Italian Ministry of Health.