Growth in serum of Pasteurella multocida and related species in chicken, turkey, duck and pig sera were compared, and selected serum-resistant and serum-sensitive strains were inoculated into 18-week-old layers. Eighty-seven field strains of Pasteurella spp. and nine reference strains representing different clones defined by restriction endonuclease analysis ( REA) profiles were used in the study. Serum activity was measured by changes in the optical density ( OD) of the serum after inoculation and incubation at 41°C for chicken, turkey and duck serum and 39°C for pig serum. Serum activity was measured by comparison with previously determined serum-resistant ( P-1059) and serum- sensitive ( CU vaccine ) strains, and classified into highly serum-resistant, moderately serum-resistant and serum-sensitive. Strains of the same REA type were found to have identical growth curves and the same maximum OD values when tested in serum from the same host species. Turkey serum was shown to be less inhibitory to a wide range of P. multocida strains than chicken, duck and pig sera. Serum- resistant strains were demonstrated among avian as well as mammalian strains. Among the avian strains, the proportion of serum-resistant strains was higher in outbreak strains than in strains from apparently healthy carriers. Removal of the capsule from selected strains by hyaluronidase treatment failed to change the serum activity. The most severe lesions in experimentally infected chickens were produced by a serum-resistant strain; however, lesions were also found in chickens infected by serum- sensitive strains, indicating the involvement of multiple factors in the virulence of P. multocida. Further investigations on serum resistance are indicated in order to relate other host and bacterial factors responsible for the development of fowl cholera.