We have examined several AT-related lesions in lymphoblastoid cell lines (LCLs) derived from AT patients. Diminished sensitivity to gamma-irradiation was found in six of seven AT-LCLs. A seventh line, from a patient with apparently normal T-cell immunity, responded normally following radiation. Constitutive proteins from exponentially growing AT-LCLs were assessed by SDS-PAGE analysis and did not differ significantly from normals. IgM synthesis was also normal except for one AT-LCL that contained native IgM molecules of different sizes, corresponding to the presence of pentamers and oligomers. Analysis under reducing conditions showed normal-sized secretory mu-chains. Finally, we examined mRNAs corresponding to two oncogenes, c-myc and c-myb, in AT and normal LCLs and found marked overproduction of c-myc in one AT-LCL (ie,, ATL6). The latter findings suggest that AT cells might be prone to aberrantly express cellular oncogenes as a result of chromosomal instability and consequent transposition of oncogenes.