Whole cells and lipopolysaccharides (LPS) of 10 isolates of Pasteurella multocida from laboratory rabbits were subjected to chemical and serological analysis. LPS of most of these isolates possessed pyrogenic potency comparable to LPS from Salmonella minnesota 9700, although their average ketodeoxyoctonate content was only 18% of that of salmonella. A gel diffusion precipitin test for somatic antigens extracted in a formal-saline solution demonstrated several isolates with three to four somatic antigens, with some variation in the major somatic type from one test to another. Conversely, the use of LPS as antigen in the gel diffusion precipitin test (i) eliminated cross-reactivity with reference antisera and (ii) often resulted in the organism being typed as serotype 12 even when the type 12 antigen was a minor antigen in the formal-saline extracts. Antisera from specific pathogen-free rabbits immunized with either whole cells or LPS of two isolates were tested against whole cells of LPS of the 10 isolates by enzyme immunoassay and indirect hemagglutination. Both whole cells and LPS of one of the isolates (isolate 2) were serologically specific, whereas those of the other isolate (isolate 1) were moderately to strongly cross-reactive with other isolates. The data indicate that although LPS is the major antigen responsible for typing based on the gel diffusion precipitin test, substances other than LPS (probably capsular polysaccharide) are responsible for the type specificity that forms the basis for the A, B, D, or E classification of this organism.