Rearrangement of the separated V, D and J segments of immunoglobulin heavy and light chain genes is an ordered and regulated process essential for the production and diversification of antibodies in mammals. Only one allele of the heavy and light chain gene locus is found functionally rearranged in normal B cells. The second allele is either non-functionally or incompletely rearranged, a phenomenon known as allelic exclusion. Recently, we and others have shown that expression of a rearranged mu gene introduced into the germline of mice leads to inhibition of rearrangement of endogenous heavy chain genes. It has been suggested that mu also exerts a positive influence on kappa light chain gene rearrangement. Here we show that a functionally rearranged delta gene is also able to prevent endogenous heavy chain gene rearrangement and could promote kappa light chain gene rearrangement.