The production of serotype A extracellular polysaccharide is thought to be associated with expression of an approximately 40-kDa lipoprotein (Plp- 40) present on the outer surface of Pasteurella multocida strains of avian origin. The tendency of certain strains to undergo colonial dissociation concomitantly with serial passaging on laboratory growth media was exploited to derive two variant strains exhibiting the capsule-deficient phenotype from a heavily capsulated parental strain. Assessments of colonial consistency, iridescence, gentian violet binding, and hyaluronidase sensitivity were consistent with cellular observations indicating little or no capsulation of derivative strains. Fluorographic analysis of electrophoretically resolved cellular lipoproteins labeled with [3H]-palmitate revealed capsular loss occurred with a concomitant diminution of Plp-40 production in the variant strains. In contrast, a phenotypically stable strain that did no undergo colonial dissociation under identical conditions exhibited no decrease in Plp-40 content. This work provides a model system for investigating the role of extracellular polysaccharide in the cell surface physiology and pathogenicity of P. multocida. The present results strongly support the notion that Plp-40 is associated with serotype A capsular material and suggest coordinate regulation of their biosynthesis.