Immunoglobulin genes are generated during differentiation of B lymphocytes by joining gene segments. A mouse pre‐B cell contains a functional immunoglobulin heavy‐chain gene, but no light‐chain gene. Although there is only one heavy‐chain locus, there are two lightchain loci: κ and λ.It has been reported that κ loci in the germ‐line configuration are never (in man) or very rarely (in the mouse) present in cells with functionally rearranged λ‐chain genes. Two explanations have been proposed to explain this: (a) the ordered rearrangement theory, which postulates that light‐chain gene rearrangement in the pre‐B cell is first attempted at the κ locus, and that only upon failure to produce a functional κ chain is there an attempt to rearrange the λ locus; and (b) the stochastic theory, which postulates that rearrangement at the λ locus proceeds at a rate that is intrinsically much slower than that at the κ locus. We show here that λ‐chain genes are generated whether or not the κ locus has lost its germ‐line arrangement, a result that is compatible only with the stochastic theory.