The present study was performed to analyze whether marginal zone B (MZ-B) cells in nondeliberately immunized adult rats are selected on basis of the specificity of their B cell receptor, and to determine to what extent memory B cells contribute to the MZ-B cell subset. To this end, the Ig PC7183 V H gene repertoire was studied among V H DJ H-μ transcripts expressed in four sequential stages of B cell development, of two individual untreated adult rats. B cell subsets, ie, pro/pre-B cells and newly formed B (NF-B) cells from bone marrow, and recirculating follicular B cells and MZ-B cells from spleen were sorted by flow cytometry. In addition, from one these rats, cells were microdissected from follicular and MZ areas of the spleen and productive PC7183 V H gene rearrangements were analyzed for the presence of somatic mutations. Sequence analysis reveals that most MZ-B cells in the adult rat, either defined by flow cytometry or by their anatomical location in the spleen, express germline encoded V H genes (naive MZ-B cells) and a minor fraction (about 20%) of the MZ-B cells carry somatic mutations (memory MZ-B cells). In addition, we show that naive MZ-B cells are a selected population of cells, both based on PC7183 V H gene repertoire and on the length of the Ig heavy (H) chain complementarity-determining region 3 (H-CDR3) region, ie, PC7183 V H DJ H-μ transcripts of MZ-B cells carry significantly shorter H-CDR3 regions than other B cell subsets.