PCR analyses of the κL chain locus in single B-lineage cells of wild-type, Cκ-, or JCκ-deficient homozygous or heterozygous mice often detect multiple in- and out-of-frame rearrangements at the κL and λL loci. They are most frequent in small pre-BII cells and equally so in wild-type and κL chain–deficient cells. Hence, κL chain production appears not to inhibit secondary rearrangements. Around 20% of all small preBII cells express IgL chains in their cytoplasm. Cells with a first productive rearrangement on one allele are favored to enter the immature B cell compartment. Thus, allelic exclusion might be secured by control of accessibility of IgL chain loci for rearrangement and by rapid selection of cells with a fitting over those with a nonfitting IgL chain.