It has long been known that testes arise only in the presence of a Y chromosome. The existence of a hypothetical gene, the testis-determining factor (TDF) on the Y chromosome, has therefore been postulated. SRY is the gene located in the sex-determining region of the human Y chromosome and has many of the properties expected for TDF. To evaluate the value of SRY PCR in sex determination, individuals with ambiguous genitalia were selected for study. By identification of the DNA sequence of SRY, a 609-bp fragment of PCR product, sex determination could be successfully made.