Fibroblasts from a patient with Fanconi's anemia were reported to show a defective excision of pyrimidine dimer [15]. We developed a sensitive radioimmuno assay which is specific for thymine dimer, the main ultraviolet photoproduct, and reinvestigated the thymine dimer excision in fibroblasts from patients with Fanconi's anemia. The analysis of 7 Fanconi's anemia cell lines did not agree with the claim mentioned above that was derived from only one Fanconi's anemia cell line. All cell lines we studied, including the cell line used previously [15], excised thymine dimer from their DNA with excision rates similar to those of normal fibroblasts. Additionally, in two Fanconi's anemia and in two normal fibroblast cell lines the repair capacity was examined.