The identification of semen is often an important issue in the investigation of rape and other crimes involving sexual assault. The most commonly used procedures for semen identification focus on the detection of sperm or of prostatic acid phosphatase activity. Methods involving the detection of spermine, choline, or semen antigens are used less often. None of these procedures is without significant problems. The identification of semen specific proteins would greatly enhance the characterization of semen. In this research, the identification of potential semen specific proteins was accomplished by comparing the proteins of human seminal plasma to the proteins of other physiological fluids by electrophoresis on polyacrylamide gels containing sodium dodecyl sulfate. Proteins found not to be present in fluids or secretions other than semen were designated p41 and p30. The protein p30 appears to be the most likely candidate for further testing as a semen marker. This protein appears to be synthesized in the prostate gland, from which it is secreted into semen. The regulation of its synthesis and secretion is not known, so further studies are needed to determine whether its synthesis is restricted to the prostate. At the present limits of detection, p30 has not been found in any other tissue or secretion. The immunological test for p30 is not interfered with by any of the nonbiological materials thus far tested. More sensitive assay methods are needed to verify these indications of specificity. Fourteen references and three illustrative figures are provided.