The primary antibody response to phosphorylcholine (PC) is dominated by T15 antibodies. There are three families of anti-PC antibodies which can be made in mice: T15, 511, and 603. All use the same H chain V, D, and J segments, but each anti-PC family has a different L chain, as well as a family-specific Vh-D junctional sequence. Here we test the hypothesis that T15 antibodies are dominant because the prototypic T15 V-D junction is generated in pre-B cells more often than the alternative non-T15 V-D junctional sequences. Rearranged IgH genes from DNA derived from fetal or newborn liver pre-B cells and from adult bone marrow pre-B cells of BALB/c mice were amplified by polymerase chain reaction, cloned, and sequenced. DNA from adult splenic B cells was also amplified, for comparison. All V1-DFL16.1 and DFL16.1-Jh1 junctional sequences were analyzed. Fifty-three percent (9/17) of all neonatal pre-B cell V-D junctions with V1 and DFL16.1 had the prototypic T15 junctional sequence, which has no N regions. In contrast, no prototypic T15 V-D junctions were observed in adult pre-B cells, and each junctional sequence was unique. Adult splenic B cells contained an intermediate number of T15-type V-D junctional sequences (7/21). The prototypic D-J junctional sequence used in many anti-PC antibodies was also observed in a high percentage of sequences. The high frequency of T15 junctions in the neonatal pre-B cells can be explained by two observations: 1) N regions are absent in neonatal but not adult junctions and 2) in the absence of N regions, joining of V, D, and J segments may be targeted to short regions of sequence homology near the ends of the genes. This mechanism would preferentially give rise to the T15 V-D and D-J junctions. Preservation of the T15 V-D junction in adult splenic B cells is most likely due to antigenic stimulation of long lived precursors, because a high frequency of T15-type D-J junctions are coexpressed with T15 V-D junctions in splenic sequences. These results predict that T15 anti-PC precursors would be made at a very high frequency in the neonate, and at a much lower frequency in the adult. This may explain why the neonatal period is critical in establishing T15 dominance.