The application of an enzyme-linked immunosorbent assay (ELISA) for 19-OH Prostaglandin F1 alpha/F2 alpha (19-OH PG F) to casework analysis of seminal contamination of swabs and stains is reported. The results are compared to those where the identification of semen was based on the presence of acid phosphatase and spermatozoa. Five hundred and one samples were analysed and there was good agreement between the results of ELISA and conventional methods. The determination of 19-OH PG F confirmed both the presence of semen where spermatozoa were absent and indicated semen was present when acid phosphatase and spermatozoa were both absent. The results indicate that 19-OH PG F represents a useful marker for the casework identification of semen and is particularly valuable where spermatozoa are absent.