The expression of three EBV open reading frames (ORF's), BBRF3, BILF1 and BMRF2 in Epstein-Barr virus (EBV)-transformed B lymphocytes from ataxia-telangiectasia (A-T) homozygotes, was studied. A-T is a human recessive genetic disorder which predisposes homozygotes and heterozygotes to cancer. Computer analysis (Robson-Garnier) was used to study the secondary structure of EBV ORF's. Three ORF's (BBRF3, BILF1 and BMRF2) found by the Kyte and Doolittle computer method to have multiple hydrophobic domains in the putative polypeptides were selected, and the polypeptides were selected, and the respective cloned EBV DNA fragments were used as probes to detect mRNA in the normal and L-6 A-T lines that was not present in the L-15 A-T line. The probe for BILF1 detected two mRNA species (3.7 and 2.0 kb) in the normal lymphoblastoid and A-T L-6 lines, while only the 3.7 kb mRNA was expressed in the A-T L-15 lymphoblasts. The probe for BMRF2 detected two mRNA species (3.7 and 2.1 kb) in the EBV-transformed normal lymphoblasts and in one A-T line (L6). The BMRF2 mRNAs were not detected in the other A-T line (L-15). This study indicated that regulation of the three EBV genes in two EBV-transformed A-T lymphoblastoid lines, differs from that in the EBV-transformed normal lymphoblastoid line. In the A-T line L-6, the three EBV genes were expressed as in EBV-transformed lymphoblastoid cells originating from a normal donor (L-21) and in the P3HR1 Burkitt's lymphoma cell line. The A-T line L-15 differed from L-6 and the other cell lines in that it expressed only one (3.7 kb) RNA species from BILF1 ORF, while ORFs BBRF3 and BMRF2 were not expressed. Since A-T L-15 line contains EBV DNA genomes, and EBV VCA is not present in these cells prior to or after TPA treatment, it is suggested that EBV gene expression is regulated by these A-T lymphoblastoid cells in a manner different from that which operates in other EBV transformed cell lines.