Immunoglobulin rearrangement is central to generating antibody diversity because of heterogeneity generated during recombination by deletion or addition of nucleotides at coding joints by the recombinase machinery. Examination of these junctional modifications revealed that the addition of nongermline-encoded nucleotides was more prevalent in adult versus fetal B cells, thus partially limiting the fetal antibody repertoire. In contrast, deletion of nucleotides occurs equivalently in B cells at different stages of development and at different points in B cell ontogeny. Finally, the bias in murine immunoglobulins for one D_H segment reading frame occurs at the D_HJ_H intermediate.