Teat lesions, produced in ewes by an experimental chapping procedure, were found to facilitate experimental infection with Mannheimia haemolytica, as assessed by observations on infection of the teat skin, teat duct and mammary gland, and on the production of mastitis. The origin of the M. haemolytica strain used (ovine tonsillar or mammary infection) did not appear to influence the results. In a second experiment, in which ewes continued to suckle their lambs but were not deliberately infected, chapping was shown to favour infection by Staphylococcus epidermidis, Staphylococcus aureus and M. haemolytica.