A case previously described as an XXXxY male has been re-appraised using 3H-thymidine. A balanced translocation probably involving the long arms of chromosomes 4 and 11 was found; all four X chromosomes were normal. Without quantitative autoradiography, this case would not have been correctly evaluated. The translocation was carried only in the balanced form. The translocated segment did not alter its labelling behaviour in its new site. Apart from the propositus, who had XXXXY sex chromosomes and the typical clinical features of the XXXXY syndrome, a brother and the father, both healthy, had the autosomal translocation. Pedigree and blood group studies showed no apparent abnormalities in the family.