Membrane-associated cross-protection factor(s) (CPF) of in vivo-grown Pasteurella multocida were solubilized by detergent and partially purified by sequential chromatography on ion exchange, gel filtration, and hydroxyapatite columns. The CPF activity was determined by challenge of turkeys vaccinated with different chromatography fractions and challenge of poults passively immunized with serum from the same vaccinated turkeys. Analyses of different chromatography fractions by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblots suggested that proteins in the range of 59-65 kDa and a protein of 39-kDa molecular mass were associated with cross-protection. Although CPF activity was found to be associated with protein-containing material, treatment with periodate diminished cross-protection, indicating that carbohydrate determinants may also contribute to immune cross-protection.