SEMINAL PLASMA-the fluid in which spermatozoa are ejaculated-has been known to inhibit reversibly both RNA and protein synthesis in sperm cells1-3. Here we report the isolation from seminal plasma, purification to homogeneity, and characterisation of a protein which we have called 'seminalplasmin', that specifically inhibits rRNA synthesis in Escherichia coli and is highly antimicrobial.