B‐1 and B‐2 cells are lymphocyte populations that differ in development, surface marker expression, tissue localization, and function. Though mainly found in the spleen, lymph nodes, and circulation of mice, small numbers of B‐2 cells are found in the peritoneal cavity, a site predominantly populated by B‐1 cells. Here, we characterized peritoneal B‐2 cells, and determined their relationship to B‐1 cells. We found that peritoneal B‐2 cells appear to be intermediate between splenic B‐2 and peritoneal B‐1 cells in terms of surface marker expression of B220, CD80, and CD43, expression of several marker genes, and in vitro viability and IgM secretion. Adoptive transfer of peritoneal B‐2 cells into severe combined immunodeficiency mice resulted in the acquisition of a phenotype reminiscent of B‐1b cells, as shown by up‐regulation of Mac‐1 and CD43, and down‐regulation of CD23. Moreover, adoptively transferred peritoneal B‐2 cells recapitulated B‐1 cell function by producing natural IgM in recipient mice. These data suggest that peritoneal B‐2 cells express some characteristics of B‐1b cells and that this similarity increases with additional time in the peritoneal cavity.