The case of a phenotypic male with a predominantly 46,XX karyotype is presented. Examination of quinacrine mustard stained frozen sections of testicular tissue revealed that 29% of Sertoli cells contained a brightly fluorescent spot, presumably representing the Y chromosome. Interestingly, these cells were the only X chromatin negative cells observed in the testis. It is concluded that it is hazardous to regard any case as a pure `XX male' without extensive study of all available tissues, including fluorescence of testicular tissue.