Human B cell subpopulations can be distinguished by the expression of B cell-associated antigens. mAb directed against these structures allow the isolation and subsequent functional analysis of such subsets. Blood B cells from healthy individuals were separated on basis of the expression of the antigens recognized by the mAb HB4 and FMC7. The B cells present in the thus obtained populations were analyzed for their ability to secrete IgM and IgG after stimulation in vitro with polyclonal B cell activators (PWM, Staphylococcus aureus Cowan I), antigens (tetanus toxoid, type 4 pneumococcal polysaccharides), and soluble B cell differentiation factors. The results suggest that B cells capable of Ig and anti-tetanus toxoid or anti-type 4 pneumococcal polysaccharide antibody production after in vitro culture are localized in a relative small B cell subpopulation carrying the FMC7 determinant but lacking HB4. This holds true for both the IgM- and IgG-secreting B cells. These data further suggest that the majority of B cells found in the blood and which can be characterized as being surface IgM+/IgD+ HB4+ are immature cells unable to respond to differentiation-inducing signals.