The T-cell receptor beta-chain gene complex contains a duplication of D beta, J beta, and C beta gene segments in mice and man. When DNA from many inbred strains of mice was screened an unusual allele of the beta locus was identified in New Zealand White (NZW) mice. This allele is distinguished by the deletion of an 8.8-kilobase segment of DNA containing C beta 1, D beta 2 and the J beta 2 cluster. Despite the fact that all NZW T-cell receptors must be derived from a single set of beta-chain gene segments, this strain has functional T cells and is phenotypically normal. This deletion of T-cell receptor beta-chain segments occurs in a strain known to contribute to lupus-like autoimmune disease.