To assess the extent and nature of somatic categorical selection of CDR-3 of the Ig H chain (CDR-H3) content in peritoneal cavity (PerC) B cells, we analyzed the composition of VH7183DJCμ transcripts derived from sorted PerC B-1a, B-1b, and B-2 cells. We divided these sequences into those that contained N nucleotides (N+) and those that did not (N -) and then compared them with sequences cloned from sorted IgM +IgD+ B cells from neonatal liver and both wild-type and TdT-deficient adult bone marrow. We found that the PerC B-1a N- repertoire is enriched for the signatures of CDR-H3 sequences present in neonatal liver and shares many features with the B-1b N- repertoire, whereas the PerC B-1a N+, B-1b N+, and B-2 N+ repertoires are enriched for adult bone marrow sequence signatures. However, we also found several sequence signatures that were not shared with other mature perinatal or adult B cell subsets but were either unique or variably shared between the two or even among all three of the PerC subsets that we examined. These signatures included more sequences lacking N nucleotides in the B-2 population and an increased use of DH reading frame 2, which created CDR-H3s of greater average hydrophobicity. These findings provide support for both ontogenetic origin and shared Ag receptor-influenced selection as the mechanisms that shape the unique composition of the B-1a, B-1b, and B-2 repertoires. The PerC may thus serve as a general reservoir for B cells with Ag binding specificities that are uncommon in other mature compartments. Copyright © 2010 by The American Association of Immunologists, Inc.