To determine directly whether B cell precursors of adult origin are capable of generating CD5+ B cells, we reconstituted neonatal C3H.SCID mice with adult C57BL/6 bone marrow and analyzed splenic B cells 10 months later. Surface staining and flow cytometry revealed that the B cells were of donor origin and that 30% were CD5+. This confirms that in vivo generated CD5+ B cells can be adult derived. After anti-IgM (but not lipopolysaccharide) stimulation in vitro, virtually all of the B cells from the bone marrow-reconstituted mice expressed surface CD5. Sequence analysis of expressed VHDJH genes from the CD5+ B cells present after anti-IgM stimulation revealed a high frequency of N nucleotide addition in CDR3 regions. The presence of N nucleotides indicates that these sequences were derived from CD5+ B cells of adult origin rather than from long-lived fetal precursor B cells present in either the adult bone marrow at the time of transfer or adult spleen. These experiments demonstrate conclusively that adult bone marrow contains precursors for CD5+ B cells and that unlike fetal liver-derived precursors these express terminal deoxynucleotidyl transferase.