Attempts were made to identify menstrual blood by means of paper electrophoresis with preparation of extracts of menstrual blood isolated under various conditions and mixed with human fibrin. Also similar analyses were conducted with blood aspirated from the median cubital vein of a woman during menstruation as well as from a man as the control, also with extracts of lochial blood from a woman after normal delivery, and of the blood obtained at arrtificial abortion. Animal fibrins (from rabbit, mouse, steer, and guinea pig) were also used to see the lytic action of the bloods. The following are the results of the present experiments. 1. The identification of menstrual blood by means of paper electrophoresis is a simple method in legal medicine and its electrophorogram is an excellent method to offer an evidence of proof for mentrual blood. 2. By this method it is possible to identify the menstrual bloodstain even after the lapse of time as much as 6 months. 3. It is possible to identify even putrefied menstrual bloodstain. 4. In the case where the material stained with menstrual blood is found in water, it is not possible to identify the menstrual blood by this method. 5. When the menstrual blood is heated at 60°C over 30 minutes, it becomes impossible to identify it by this method. 6. In the case of venous blood during menstruation fibrinolytic product can be detected only on the first day of menstruation, but since it appears only in trace, it is easy to differentiate it from menstrual blood. 7. As for lochial blood the fibrinolytic product can be detected only in the blood obtained on the first and second days of puerperium, but the amount being so slight that it can readily be distinguished from menstrual blood. 8. In the case of the blood obtained at artificial abortion fibrinolytic product appears just as much as in the case of menstrual blood, and thus it is impossible to differentiate it from mentrual blood by this method. 9. As for the use of human fibrin it is best to employ it while it isdresh, but the human fibrin up to 6 days old can be used. However, the older is the human fibrin the lesser the fibrinolytic product detectable. 10. In the case using animal fibrins mixed with the extract of menstrual blood some do produce fibrinolytic product in trace, but since there is a danger of also producing the fibrinolytic product-like substance in venous blood, it is advisable not to use animal fibrins.