T-cell mediated and humoral responses directed to microbial antigens were investigated, at the time of the initial visit, in a group of 139 patients with HIV-1-related persistent generalized lymphadenopathy (PGL) enrolled in a longitudinal study. In vivo and in vitro cell-mediated responses to tuberculin were lower in patients than in controls. Differences were not significant for candidin and streptococcal antigen in vitro, whereas higher responses were observed in the patient group for cytomegalovirus antigen. Following immunization, a subgroup of patients did not have a significantly raised serum antitetanus antibody level, whereas in vitro lymphocyte proliferative responses to tetanus toxoid were lower than in controls. No association was found between these abnormalities and other immunological parameters, including the blood level of CD4+ lymphocytes. Lower responses to most microbial antigens were observed in patients with HIV-1-related symptoms in addition to lymphadenopathy, or the patients who progressed to AIDS in the 2 years following the study. Moreover, intravenous drug users showed higher responses than homosexual patients, possibly because of the influence of previous infections on immunological responses to microbial antigens.