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The study of murine memory B cells has been limited by small cell numbers and the lack of a definitive marker. We have addressed some of these difficulties with hapten-specific transgenic (Tg) mouse models that yield relatively large numbers of antigen-specific memory B cells upon immunization. Using these models, along with a 5-bromo-2′-deoxyuridine (BrdU) pulse-label strategy, we compared memory cells to their naive precursors in a comprehensive flow cytometric survey, thus revealing several new murine memory B cell markers. Most interestingly, memory cells were phenotypically heterogeneous. Particularly surprising was the finding of an unmutated memory B cell subset identified by the expression of CD80 and CD35. We confirmed these findings in an analogous V region knock-in mouse and/or in non-Tg mice. There also was anatomic heterogeneity, with BrdU⁺ memory cells residing not just in the …