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