Lactating Welsh Mountain ewes were inoculated, 3 weeks after lambing, with between 1000 and 10,000 colony forming units of a number of isolates of Pasteurella haemolytica. Isolates from severe, acute mastitis in a ewe, from ovine and bovine pneumonic lesions and from the nasal cavity of healthy lambs, all gave rise to severe, acute mastitis that was clinically indistinguishable from that seen naturally. Two isolates from the milk of ewes with subclinical mastitis did not cause clinical disease after inoculation and, in most ewes, were immediately eliminated. These results suggest that a variety of strains of P. haemolytica are capable of causing severe mastitis in sheep, regardless of their origin, and that there are strains of lower pathogenicity for the mammary gland which are not capable of causing clinical mastitis.