Since P. anatipestifer (PA) serotypes 1, 2 and 5 gave no significant protection against heterologous serotype challenge, a trivalent formalin-inactivated bacterin containing all these serotype cells was developed and tested in the laboratory. Two inoculations of bacterin in White Pekin ducklings at 10 and 17 days of age gave highly significant protection against challenge with virulent organisms. The protection lasted only 2 wk after the 2nd inoculation. A third bacterin inoculation given at 31 days or exposure to an active field infection during the period of protection extended protection to market age (7 wk). Aluminum-hydroxide-gel-adsorbed bacterin gave no better protection than bacterin without adjuvant. One inoculation of multiple oil-emulsion (MOE) bacterin gave absolute protection up to market age; unfortunately, it produced noticeable tissue reactions at the site of inoculation. Day-old ducklings were also responsive to PA bacterin and developed significant immunity against experimental challenge.