Polypeptides from whole cell preparations of Pasteurella multocida serotypes A:12 and A:3 were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose paper. Antigens were detected by immunoblot analysis, using sera from 3 groups of rabbits. Sera were obtained from rabbits inoculated intranasally with P multocida serotype A:12 or A:3, from rabbits maintained in a rabbitry with enzootic P multocida A:12 infection, and from rabbits maintained in a rabbitry with enzootic P multocida A:3 infection. Immunoblot analyses of pre- and postinoculation sera from experimentally infected rabbits, using serotype A:12 antigen, revealed 3 polypeptides with approximate molecular mass of 28, 30, and 37 kDa that consistently detected antibodies after P multocida-induced infection. Sera from rabbits naturally infected with either serotype, tested against serotype A:12 and A:3 antigens, detected the same polypeptides in both serotypes. Thus, immunologic reactivity to these polypeptides may be useful for serologic detection of P multocida infection.