The genes that encode the immunoglobulin proteins made by B lymphocytes are made up of segments that are separately encoded in the germ-line genome and brought together by recombination during B-cell ontogeny. There are two types of immunoglobulin light chain, kappa and lambda, but only a single type is expressed in individual B cells. It is thought that kappa gene recombination precedes lambda gene recombination during B-cell ontogeny. We describe here unusual recombinations that have occurred in two lambda-producing B-cell lines and suggest that they are involved in the developmental switch from kappa to lambda gene expression in maturing B cells. These recombinations involve the J kappa-C kappa introns of V-J joined but nonfunctional kappa genes and a sequence that in the germ line occurs downstream of the C kappa exon (called RS, for recombining sequence).