Human spleen T cells stimulated with pokeweed mitogen (PWM) were cloned under limiting conditions in microculture systems using interleukin 2 and irradiated autologous cells. Clones were screened for helper or suppressor activity on PWM-dependent B cell differentiation by adding cell aliquots to either isolated B cells and PWM or to mixtures of T and B cells and PWM. Out of 97 clones tested, 14 promoted intense B cell differentiation, as assessed by measurements of secreted IgG, and 6 strongly inhibited B cell maturation induced by spleen T cells. All the selected clones maintained their original activity after short-term clonal expansion; in addition, a similar (helper or suppressor) effect was detected when the total number of plasma cells per well was evaluated. Suppressor clones had no cytolytic activity on autologous T and B cell blasts, K562 cells or antibody-coated L 1210 mouse cells. Nine helper and 6 suppressor clones were analyzed for a battery of surface markers. All the clones were E rosette-positive and expressed HLA-DR (Ia) antigens. Fcmu receptor was present on a single helper clone, whereas Fc gamma receptor was expressed on a suppressor clone only. All but two clones expressed the OKT4+/ OKT8-, a single suppressor clone was OKT8+/OKT4-, whereas coexpression of OKT4 and OKT8 antigens was detected in one helper clone. Thus, the claim that helper T cells are OKT4+/OKT8- and suppressor T cells are OKT4-/OKT8+ is not supported by the analysis of their phenotype at the clonal level.