Peritoneal cavity (PerC) B-1 cells have long been known to express CD11b, which is coexpressed with CD18 to form the Mac-1/CR3 complement receptor and adhesion molecule. However, although all PerC B-1 cells are commonly believed to express CD11b, we show here that nearly half of the cells in each of the PerC B-1 subsets (B-1a and B-1b) do not express this surface receptor. The CD11b⁺ cells in each B-1 subset are larger and more granular and express higher levels of surface IgM than the CD11b⁻ B-1 cells. In addition, the CD11b⁺ B-1 cells initiate the formation of tightly associated doublets that are present at high frequency in adult PerC. Finally, and most importantly from a developmental standpoint, the CD11b⁺ B-1 cells have a limited reconstitution capability: when sorted and transferred into congenic recipients, they reconstitute their own (CD11b⁺) B-1 subset but do not reconstitute the CD11b⁻ B-1 subset. In contrast, CD11b⁻ B-1 cells transferred under the same conditions efficiently replenish all components of the PerC B-1 population in appropriate proportions. During ontogeny, CD11b⁻ B-1 cells appear before CD11b⁺ B-1 cells. However, the clear phenotypic differences between the neonatal and adult CD11b B-1 subsets argue that although CD11b⁻ B-1 give rise to CD11b⁺ B-1 in both cases different forces may regulate this transition.