Receptor editing, clonal deletion, and anergy are the mechanisms by which B cells maintain tolerance to self antigens. To determine the extent to which receptor editing shapes the normal antibody repertoire, we generated an immunoglobulin kappa polymorphism that facilitates the detection of;editing of immunoglobulin light chains in vivo. We found that B cells are targeted for editing during a 2-hour delay in development at the pre-BII cell stage, and that about 25% of all antibody molecules are produced by gene replacement. These results suggest that receptor editing represents a major force in shaping the antibody repertoire.