Anionic and cationic proteins were isolated from cervical mucus obtained from the cow during oestrus. The anionic proteins showed no antimicrobial activity whereas the cationic proteins inhibited the growth of Staphylococcus aureus S305 and Brucella abortus S19. Polyacrylamide gel electrophoresis, immunoelectrophoresis and isoelectric focusing experiments revealed the heterogeneous nature of the cervical mucus proteins. At pH 3·0, four main cationic fractions could be resolved by disc acrylamide electrophoresis. By electrophoresis in agarose at pH 8·6, aggregation occurred, with the exception of a more basic fraction which migrated towards the cathode. Lysozyme was absent from the isolated soluble proteins of cervical mucus, but serum albumin, and β- and γ-globulins were detected. Antimicrobial proteins in cervical mucus may provide an initial line of defence for the uterus against invading pathogens.