It has been shown that the X-ray-sensitive Chinese hamster V79 mutants (V-E5, V-C4 and V-G8) are similar to ataxia-telangiectasia (A-T) cells. To determine whether the AT-like rodent cell mutants are defective in the gene homologous to A-T (group A, C or D), human chromosome 11 was introduced to the V-E5 and V-G8 mutant cells by microcell-mediated chromosome transfer. Forty independent hybrid clones were obtained in which the presence of chromosome 11 was determined by in situ hybridization. The presence of the region of chromosome 11q22-23 was shown by molecular analysis using polymorphic DNA markers specific for the ATA, ATC and ATD loci. Seventeen of the obtained monochromosomal Chinese hamster hybrids contained a cytogenetically normal human chromosome 11, but only twelve hybrid cell lines were shown to contain an intact 11q22-23 region. Despite the complementation of the X-ray sensitivity by a normal chromosome 11 introduced to A-T cells (complementation group D), these twelve Chinese hamster hybrid clones showed lack of complementation of X-ray and streptonigrin hypersensitivity. The observed lack of complementation does not seem to be attributable to hypermethylation of the human chromosome 11 in the rodent cell background, since 5-azacytidine treatment had no effect on the streptonigrin hypersensitivity of the hybrid cell lines. These results indicate that the gene defective in the AT-like rodent cell mutants is not homologous to the ATA, ATC or ATD genes and that the human gene complementing the defect in the AT-like mutants seems not to be located on human chromosome 11.