Newly generating memory B cells rapidly accumulate somatic mutations that can alter their Ag-combining sites and potentially engender recognition of self determinants. To investigate the possibility that, during their emergence secondary B cells pass through a window of tolerance susceptibility, we have examined the in vitro generation of memory B cells in the presence or absence of tolerogen. The findings indicate that, before antigenic stimulation, precursors to memory B cells are resistant to tolerance induction. However, 2 to 7 days after T cell-dependent antigenic stimulation, newly emerging hapten-specific secondary B cells can be inactivated by the presence of hapten on a carrier not recognized by available Th cells. This inactivation can be blocked by the presence of free hapten and can be competed by the presence of immunogen. Inactivation of newly generating secondary B cells appears less specific than the tolerance induction of immature neonatal or bone marrow B cells because inactivation can be accomplished by cross-reactive determinants. Interestingly, the presence of tolerogen after primary stimulation did not preclude the generation of cells responsive to a third in vitro stimulation. Therefore, whereas newly emerging memory B cells are highly susceptible to inactivation, the progression of the clones of progenitors to memory B cells appears resistant to tolerance induction.