Spontaneous adherence of bacteria to human peripheral blood mononuclear cells (PBMC) represents a useful tool for analysis of lymphocyte subsets with different functions. We have recently shown that PBMC can be divided into 2 populations based on their ability to bind Salmonella minnesota R345 (Rb) bacteria. By using these procedures, here, we provide evidence that Rb-bound and Rb-unbound PBMC populations give similar proliferative responses to phytohemagglutinin (PHA) and concanavalin A (Con A), while the pokeweed mitogen (PWM)-induced proliferative and differentiative responses are higher in the Rb-unbound than in the Rb-bound PBMC fraction. Moreover, enhanced PWM-induced responses are obtained in Rb-unbound cell cultures enriched for T4+ cells. When B (non-E rosetting) cells are cultured with purified T lymphocytes from the Rb-bound (T-Rb+) and Rb-unbound (T-Rb-) fractions, comparable PWM-induced mitogenic responses are observed. The T-Rb- population contains a higher percentage of cells expressing T4+ phenotype, and when added to B cell cultures a more elevated PWM-induced IgA, IgG and IgM synthesis is observed than in B cell cultures containing T-Rb+ cells. These results suggest that the T-Rb- fraction is enriched for T cells which help IgA, IgG and IgM responses.