We isolated germinal center B cells by exploiting their high affinity for peanut agglutinin (PNA). The PNA+ and PNA- B cells, fractionated by panning on PNA-coated petri dishes, were examined for their ability to transfer memory responses to irradiated recipients at various times after priming. With such fractionated B cells from lymph nodes taken at the peak of germinal center formation, the largest response was obtained in recipients of the PNA+ B cell population. At 4 to 5 wk after priming, and 10 days after challenge with an unrelated antigen, memory responses were approximately equal in recipients of PNA+ or PNA- B cells. At 14 wk after priming, memory responses were found only in recipients of the PNA- B cell population. Memory B cells from the spleen, taken from mice primed in the footpad 8 wk earlier, were also PNA-. Finally, we show that boosting with a TNP-conjugate in the footpad, 6 mo after priming in the same footpad, induced the reappearance of marked memory responsiveness in the PNA+ B cell fraction of the draining node.