We have previously demonstrated a dramatic preference for utilization of the most JH-proximal VH gene segments in the newborn liver versus adult spleen. We now examine in detail the relative expression of different VH gene families throughout ontogeny and in immunodeficient mice to gain insight into factors that cause the shift in VH usage. We find that the relative expression of VH gene families remains constant and biased throughout fetal and neonatal liver development. In addition, the primary VH repertoire expressed in neonatal spleen displays a similarly biased, position-dependent VH repertoire. The pattern of VH gene expression begins to change at 5-7 d postnatally and reaches the adult randomized pattern at approximately 2 wk of age. We also find biased expression of JH-proximal VH gene families in adult bone marrow and in spleens of adult leaky scid mice, suggesting that the spontaneously generated repertoire of adult mice is similar to that observed in neonates. Together, these data suggest that a position-dependent repertoire is generated in differentiating pre-B cells at all stages of ontogeny, at least in part, as a result of preferential rearrangement of proximal VH gene segments. Therefore, mechanisms subsequent to V gene rearrangement, such as regulatory interactions and antigen selection, must play a major role in normalizing the repertoire.