During B lymphocyte development, pro-B cells that fail to rearrange an immunoglobulin heavy (IgH) chain allele productively are thought to undergo developmental arrest and death, but because these cells are short-lived in vivo they are not well characterized. Transgenic mice expressing the apoptosis regulatory gene bcl-x_L in the B lineage developed large expansions of pro-B cells in bone marrow. V(D)J rearrangements in the expanded population were nearly all nonproductive, and DJ_H rearrangements were enriched for joints in D_H reading frame 2 and for aberrant joints with extensive D_H or J_H deletions. Thus, the death of pro-B cells with failed immunoglobulin rearrangements occurs by apoptosis, and bcl-x_L can deliver a strong survival signal at the pro-B stage. This analysis also demonstrates that immunoglobulin gene rearrangement is less precise than previously appreciated.