Abstract An experimental, inactivated P. anatipestifer type ‘G’ vaccine was produced containing 10 9 CFU/0·5 ml and aluminium hydroxide adjuvant (25% final concentration). From comparative pathogenicity studies of field isolates type ‘A’, ‘D’ and ‘G’, the ‘G’ serotype was selected as being most virulent and suitable for challenge. Vaccination trials, in the laboratory, showed that a single subcutaneous inoculation (0·5 ml) of the above vaccine at 14 days provided the best protection against challenge up to 5 weeks of age. During the course of investigations, maternal antibodies were encountered in ducklings up to the age of 6 days and natural resistance to infection in adults from 5 weeks upwards. Evidence has also been provided of cross antigen-antibody reactions detected in the ELISA, between the ‘A’, ‘D’ and ‘G’ serotypes. A threefold ‘protective index’ system is described which provides a very sensitive measurement of effective vaccination.