Cell suspensions obtained from 54 human lymph nodes involved by different pathological conditions were characterized by conventional markers and by the OKT-3, OKT-4, OKT-8, OKIa-1, and OKM-1 monoclonal antibodies. In 18 cases of reactive lymphoid hyperplasia, the majority of lymph node cells were mature T lymphocytes (E-RFC = 56 +/- 9%; OKT-3+ = 63 +/- 10%); among T-cell subsets, OKT-4+ cells were 49 +/- 8% whereas OKT-8+ cells were 21 +/- 8% (T4/T8 = 2.7 +/- 1.1). This distribution of T-cell phenotypes was not similar in the different histological types of reactive lymphoid hyperplasia. In fact, an increase in the percentage of OKT-8+ cells (25 +/- 9%; P less than 0.05) and a decrease in the values of the T4/T8 ratio (2.1 +/- 1.0; P less than 0.05) were observed in 9 cases of reactive lymphoid hyperplasia of follicular type when they were compared to the mixed and sinus types. In 13 lymph nodes involved by B-cell lymphoma, the percentage of T lymphocytes was markedly reduced (E-RFC = 21 +/- 12%; OKT-3+ = 27 +/- 18%) and the percentage of OKIa-1+ cells (51 +/- 15%) was significantly (P less than 0.01) increased as compared to reactive nodes; in addition, in these cell suspensions, an increase in the relative proportion of OKT-8+ cells (T4/T8 = 1.4 +/- 0.7; P less than 0.01) could also be demonstrated. Finally, a clear prevalence of OKT-4+ cells on OKT-8+ cells was demonstrated in 5 cases of tuberculous lymphadenitis (T4/T8 = 3.9 +/- 1.5; NS) and in 18 cases of Hodgkin's disease (T4/T8 = 4.2 +/- 2.0; P less than 0.01). In tuberculous lymphadenitis, a significant increase (P less than 0.01) in the percentage of OKM-1+ cells could also be demonstrated.