Two XX males who were second cousins are reported. A genetic mechanism producing maleness is suggested. The putative factor had been transmitted solely through males, which excludes the possibility of a heritable X-Y interchange. Recent reports on fluorescent Y chromatin in Sertoli cells of XX males prompted investigations into the fluorescence patterns of testicular cells. Sertoli cells from three XX males displayed brightly fluorescent spots, but it was concluded that they did not represent Y chromosomes. Evidence for this conclusion was obtained from the study of testicular fluorescence in XX, XXY and XY males. No visually detectalbe cytogenetic evidence for an increase in length or altered banding pattern of one of the X chromosomes was found in three XX males. We conclude that an autosomal gene is the most likely explanation of the male differentiation in the two XX males presented here.