When various polyclonal lymphocyte activators (PLA), such as the capsular polysaccharide of Klebsiella pneumoniae (CPS-K), E. coli lipopolysaccharide (LPS), concanavalin A (Con A), dextran sulfate (DS), phytohemagglutinin (PHA), and pokeweed mitogen (PWM) were injected into mice primed with sheep red blood cells (SRBC), anti-SRBC secondary plaque-forming cell (PFC) responses in vitro of their spleen cells to SRBC and to polyclonal B cell activatory (PBA) were more or less decreased. The decrease in the responsiveness was accompanied by the decrease in the number of SRBC-specific rosette-forming cells (RFC) of B-cell type (B memory cells). This resulted neither from emigration of RFC out of the spleen, nor from change of RFC to antibody-forming cells. Further, we revealed that the decreased responsiveness occurs exclusively in the B cell-rich fraction of the spleen cells from PLA-treated SRBC-primed mice, but not in their T cell-rich fraction. It is concluded therefore that PLA exhibited a common action to reduce selectively B-memory cell function by decreasing the number of B memory cells without differentiation to their end cells, although the strength of the action of various PLA varied.