Current concepts of the structure of the capsule and cell wall of gram negative bacteria and the role of these substances on the serologic classification of bacteria were reviewed. Emphasis was placed on the methods used to serotype Pasteurella multocida as developed from studies on isolates from various animal species and the application of these methods in serotyping isolates of Pasteurella multocida from laboratory rabbits. Evidence was presented that the indirect hemagglutination test used to identify "capsular" types A, B, D, and E is based on the interaction of antibody with type specific lipopolysaccharide and is therefore a test for somatic rather than capsular materials. It also was observed that several rabbit isolates were not serologically typable by currently available reagents. Commonly used techniques to obtain relatively homogeneous bacterial extracts were discussed, and the need to characterize the extracts of Pasteurella multocida from rabbits both chemically and immunologically was emphasized.