The PCR method specifically amplified Y chromosome specific sequence and X chromosome specific sequence belonging to alphoid centromeric repeat family. X and Y specific sequences could be detected from a cotton cloth 1 millimeter square, on which blood diluted by 10,000 times had been attached. These sequences and the gender could also be detected from a bloodstain on even 1 millimeter of a single cotton fiber. The sequences could be detected from blood left at room temperature for 7 months, from bloodstains left at room temperature for 2 years, after a blood specimen was heated in a water bath at 100 degrees centigrade for 9 hours, and after bloodstains were heated in an electric furnace at 150 degrees centigrade for 30 minutes. Sex determination was possible from aged bloodstains preserved at room temperature for 22 years. Sequences could also be determined when male and female blood specimens were mixed together. Findings revealed that sex determination can take place in a simple and quick manner, even from heated or aged specimens. Therefore, PCR should be a useful technique in forensic medicine. Figures and 51 references