The relationship between infection with human immunodeficiency virus (HIV) and various immunological parameters was studied in: (a) healthy controls; (b) homosexual individuals from the AIDS risk group without anti-HIV antibodies; (c) idem, but with anti-HIV antibodies; (d) patients with persistent generalized lymphadenopathy (PGL); (e) patients with AIDS-related syndrome; (f) patients with AIDS and opportunistic infections. In each group, consisting of 15-20 individuals, the following parameters were studied: absolute numbers of CD4+ and CD8+ cells; ratio CD4+/CD8+; cellular immune responses as measured in vivo by delayed-type hypersensitivity (DTH) and in vitro; and antibody response in vivo after immunization with a low dose of keyhole limpet haemocyanin. Healthy HIV antibody-positive individuals and patients with persistent generalized lymphadenopathy already showed a decreased CD4+/CD8+ ratio, mainly due to an increase in the number of CD8+ cells. The ratio in the AIDS-related syndrome and AIDS groups was even lower, but this was now due to low numbers of CD4+ cells while the number of CD8+ cells was normal. The lymphocyte proliferative response was low in the HIV antibody-positive group, normal in the group with persistent generalized lymphadenopathy and profoundly decreased in the AIDS-related syndrome and AIDS groups. DTH was enhanced in the PGL group and diminished in both ARC and AIDS. Compared to healthy controls, the antibody response upon immunization with a low dose of keyhole limpet haemocyanin was depressed (although not absent) in all groups studied, even in HIV antibody-negative homosexuals. In the HIV antibody-positive group, the severity of the impairment of the various parameters of immunocompetence was not related to the presence of HIV antigenaemia.