SummaryImmunoglobulin and T cell receptor gene probes have been used to investigate cell lineage and monoclonality in lymphoid malignancies. In the present study we have used T cell receptor β‐and γ‐chain gene probes to screen for abnormal rearrangements of these genes in B lymphoblastoid cells from patients with ataxia‐telangiectasia (A‐T). No rearrangement of either gene was observed but deletion of a β‐chain gene allele is described for one A‐T cell line. Expression of mRNA hybridizing to the β‐chain gene probe was demonstrated for two A‐T homozygotes (brother and sister) as well as for their mother (heterozygote). This transcript was found to be truncated in all three cases.