Five homozygous patients with Fanconi's anemia and nine heterozygous relatives were studied for mutagen sensitivity by a new method. Lymphocyte cloning survival after treatment with several doses of the cross linking substance 8-methoxy-psoralene and exhaustive activation with UVa-light was used for precise measurement of the dose-response. Sensitivity of the homozygous lymphocytes was greatly increased; the degree of the cellular defects corresponded to the clinical course. In the two cases with highest sensitivity, the heterozygotes could be differentiated from normal probands by increased sensitivity, whereas this was not the case in three other families.