Sperm detection can be an important factor in confirming sexual assault in cases of rape. This paper compares two biochemical methods used in forensic medicine: the first detects the presence of zinc, the second detects acid phosphatase activity. The population studied was composed of 174 consenting women seen at the Male Infertility Center in Toulouse, France. The date of their last sexual intercourse was known accurately. Cytology was the reference test to confirm the presence of sperm in the vaginal samples. We studied the sensitivity, specificity, and positive and negative predictive value of the two biochemical methods. Acid phosphatase detection was the most valuable technique, but its use is limited in time. The zinc test gave disappointing results in our study and does not seem to be a useful reference method for the forensic physician.