A bnormal T cell function is a feature of a spectrum of inherited and acquired diseases. We have detected a frequent restriction fragment length polymorphism in the human T cell antigen receptor beta-chain locus that may aid in the analysis of these disorders. A study of a panel of 18 normal individuals, testing for the presence of the polymorphism, showed it to account for 36% of the alleles in that group. In view of the fact that the T cell receptor beta-chain locus has been mapped to chromosome 7, and that the disease ataxia telangiectasia (AT) is associated both with abnormal T cell function and with chromosomal abnormalities of the same region of chromosome 7, we investigated the possibility that the polymorphism could demonstrate linkage of the T cell receptor locus to the gene for that disease. We demonstrated that the mutation causing AT did not lie within the beta-chain locus itself, and that there was preliminary evidence that the two loci were not closely linked. This polymorphism may provide a useful tool for the study of other genetic disorders associated with abnormalities of T cell function, as well as disorders associated with inherited or acquired abnormalities of chromosome 7.