Peritoneal B1 and B2 cells efficiently reenter the peritoneal cavity following adoptive intravenous transfer. Peritoneal B cells and splenocytes were differentially labeled with CFSE and TAMRA, and mixtures containing equal numbers of B cells were injected intravenously into wild-type recipients. One day after transfer, the ratio of transferred peritoneal B1 cells to splenic B2 cells (A) and peritoneal B2 cells to splenic B2 cells (B) was determined in different compartments by flow cytometry. Cells were addressed as CD19⁺B220^lowCD23⁻ for B1 cells and CD19⁺B220^highCD23⁺ for splenic B2 cells as well as peritoneal B2 cells. Circles indicate the mean of 9 mice analyzed in 3 independent experiments performed; error bars, SEM; SPL, spleen; MLN, mesenteric lymph nodes; PLN, peripheral lymph nodes; PP, Peyer patches; PerC, peritoneal cavity. Statistical differences are indicated for the mean values and variances separately as follows: *P < .05; **P < .01; ***P < .001.