Nine pairs of Simply Basic 100 percent nylon panties with a 100-percent cotton gusset and nine pairs of Penman's Hi-Cut 100 percent cotton panties were used in this study. They were selected because they are typical of what is encountered in casework. To better simulate casework conditions, the panties were machine washed and dried before they were used in the study. The applied semen stains were allowed to dry overnight. The laundered semen stains on the panties were tested with standard tests used by the Royal Canadian Mounted Police Forensic Laboratories, and they were then subjected to DNA typing. Tests for semen identification included the Fast Blue Test for acid phosphatase, microscopic identification of spermatozoa, and the PSA ABAcard Test for detection of prostate specific antigen (PSA). DNA was extracted from the samples via a differential organic extraction, and the DNA was typed by using PCR amplification of Short Tandem Repeats (STRs). The findings showed that compared to control semen stains, laundered stains on cotton panties had reduced but significant numbers of spermatozoa remaining, and they still tested positive for the presence of PSA. Further, they tested negative for acid phosphatase, retained approximately 43 percent of their DNA, and produced complete DNA profiles. Compared to control semen stains, laundered stains on nylon panties tested negatively for PSA and acid phosphatase, retained less DNA and spermatozoa than was observed on laundered cotton panties, and did not always yield DNA profiles. Given the persistence of spermatozoa on cotton, investigators may be able to obtain the DNA typing profile of a perpetrator of sexual assault even after a victim has laundered her clothes. 3 figures, 2 tables, and 11 references