An isolate of Pasteurella haemolytica (A9), which consistently produced severe mastitis in ewes, was inoculated into the lactating mammary glands of a variety of species. Mastitis did not develop after the inoculation of log-phase bacteria into the mammary gland of lactating mice, rats, rabbits or sows but did so in the mammary gland of two cows. Another A9 isolate from a ewe with mastitis and an A1 isolate from a bovine pneumonic lung also induced mastitis in cows. Thus, in this study, P. haemolytica produced mastitis only in ruminant animals.