BURNET'S clonal selection theory¹ suggests that each B lymphocyte is committed to a single antibody specificity. This is achieved by a programme of somatic rearrangements of the gene segments encoding antibody variable (V) regions, in the course of B-cell development^2,3. Evidence from immunoglobulin-transgenic mice and immunoglobulin-gene-transfected transformed pre-B cells suggests that the membrane form of the immunoglobulin heavy (H) chain of class µ (µm), expressed from a rearranged H-chain (IgH) locus, may signal allelic exclusion of the homologous IgH locus in the cell^4–6 and initiation of light (L)-chain gene rearrangement in the Ig/c loci6. We report here that targeted disruption of the membrane exon of the µ chain indeed results in the loss of H-chain allelic exclusion. But, some K chain gene rearrangement is still observed in the absence of µm expression.