AbstractPrevious studies of the expression of SV40 genetic information by skin fibroblasts included limited numbers of cell donors and failed to adequately consider possible effects of age, sex, and ethnic origin on assay results. A population of 76 healthy subjects were selected for study following determination of personal and family disease history and karyological analysis. Skin fibroblasts from these individuals were tested for expression of SV40 T‐antigen by indirect immunofluorescent assay. The data were normally distributed and showed no significant differences between the age, sex, or ethnic groups tested. The occurrence of rare karyological anomalies in this control population had no effect on T‐antigen expression. Fibroblasts from 10 Fanconi anemia patients demonstrated significantly elevated expression of T‐antigen compared to the well‐defined control population, based on simple statistical criteria. T‐antigen expression was elevated in two young patients prior to the onset of anemia and did not appear to correlate with the incidence or severity of other specific symptoms. Thus, elevated T‐antigen expression in Fanconi anemia fibroblasts reflects an actual defect at the cellular level, rather than clinical, age, sex, or ethnic factors not previously