5′ Nucleotidase (5′NT) is an ectoenzyme associated with the plasma membrane of most mammalian cells. Low 5′NT activity has been observed in peripheral blood lymphocytes from patients with immunodeficiency states. 5′NT activity was measured in null and T-enriched lymphocytes from asymptomatic homosexual men and from 20 men with various degrees of the acquired immune deficiency syndrome (AIDS). Asymptomatic homosexuals were self-referred because of their concern about AIDS and were not necessarily representative of homosexuals in the general population. Enzyme activity was significantly decreased in both null (7.0 +/- 2.4 nmol/10(6) cells/h) and T-enriched (12.0 +/- 6.0 nmol/10(6) cells/h) lymphocytes in homosexuals as compared to lymphocytes from aged-matched heterosexual male and female controls (null = 10.8 +/- 6.5 and T = 22.3 +/- 10.6, P less than .0001 and .008, respectively). Decreased activity was present regardless of whether the patients were asymptomatic, had prodromal symptoms such as fever, lymph node enlargement, weight loss and diarrhea, or had opportunistic infections or Kaposi's sarcoma. Homosexuals had a significantly higher fraction of lymphocytes expressing the activation antigens T10 (20% +/- 3.3%) and Ia (13% +/- 2.9%) than controls (11% +/- 1.8% and 5% +/- 0.8%, respectively, P less than .05). They also had a significantly lower fraction of OKT4-positive helper lymphocytes than controls (22% +/- 3.4% v 35% +/- 2.2%, P less than .05). 5′NT activity in lymphocytes enriched for null cells from homosexuals correlated inversely with the percentage of Ia-positive lymphocytes (r = -.655; P less than .02). There was no correlation between 5′NT activity and the percentage of T4- or T8-positive lymphocytes or the T4/T8 ratio. Moreover, 5′NT activity was significantly decreased in both OKT4 (P less than .025) and OKT8 (P less than .05) enriched lymphocytes in homosexuals compared to controls. The data suggest that decreases in 5′NT may be a generalized defect of the peripheral blood T lymphocytes from active homosexuals that is independent of increases or decreases in specific T subpopulations or clinical status. It may contribute to the pathogenesis of AIDS.