In pokeweed (Pa‐1)‐stimulated human lymphocyte cultures, T cells are essential for the survival, proliferation, plasma‐cell development, and high‐rate Ig secretion of B cells. Their effects are T‐cell‐specific in the sense that B‐Cell stimulation does not take place in the absence of T cells even when fibroblasts or monocytes are provided. The experimental system is the most effective model for activation of human B lymphocytes so far described. Plasmablast development requires approximately 7 days in culture. In T + B‐cell cultures established at 1 × 106/ml (1 × 104/mm2) initial cell density, plasma cells can secrete, on the average, as much as 40–70 pg IgM or IgG per cell per day. When the initial T‐cell density is increased well above 1.0 × 106/ml, a T‐cell‐mediated depression of Ig synthesis becomes predominant. Thus, in the pokeweed model T‐cell effects represent a balance of helper and suppressor influences. The study also shows that B cells are heterogeneous. A non‐adherent IgG‐committed (smIg−?) tonsil B‐cell population seems to be less susceptible to T suppressor effects than normal tonsil B cells. This subset proliferates particularly well and synthesizes large quantities of IgG in the presence of