In most rape cases, the laboratory focuses on analyzing stains and vaginal swabs because they can provide evidence of sexual contact and genetic markers. This article outlines a basic scheme for collecting and analyzing semen evidence. The discussion concerning techniques for finding sperm as supporting evidence focuses on the quantitative acid phosphatase (ACP) assay and p30 testing. Because semen may not be found in some 25 to 30 percent of otherwise well-documented sexual assaults, the paper underscores the importance of other evidence such as torn clothing, bruises, and bite marks. Ways to estimate when intercourse occurred are explained, as is genetic typing to determine the assailant's blood group and type. Although the latter method cannot uniquely specify an individual, it significantly restricts the suspect population. The article also discusses the collection of evidence by emergency room personnel, laboratory reports, and court testimony. Charts and 22 references are supplied.