Ataxia telangiectasia (AT) is an autosomal recessive disease, characterized by both neurological disorders and a high incidence of early-onset cancers. On a cellular level, cellular radiosensitivity and radioresistant DNA synthesis are the hallmarks of AT. While expression of cellular radiosensitivity varies somewhat among affected individuals, radioresistant DNA synthesis is seen consistently and, in fact, is the only end point used for assigning individuals to genetic complementation groups. For this reason, complementation-group-specific correction of radioresistant DNA synthesis in AT cells has long been thought to be an absolute requirement for confirmation of a bona fide clone of an AT gene. Since primary AT cells grow poorly in culture, SV40-immortalized AT fibroblasts are the usual recipients of transfected DNA in these studies. In experiments reported here, we demonstrate that SV40-immortalized AT fibroblasts have significantly reduced radioresistant DNA synthesis compared to primary AT fibroblasts, and their response to radiation is more like normal cells, in that both the radiosensitive and radioresistant components appear to be present. This suggests that there may be an interaction between SV40 proteins and the AT gene product or its downstream elements. This partial "complementation" of radioresistant DNA synthesis in SV40-immortalized AT cells complicates complementation cloning strategies, and should be considered when terminally screening putative AT gene clones by analysis of radioresistant DNA synthesis.