Cells of P. haemolytica [the causal organism of enzootic pneumonia and septicemia in sheep] serotypes A1 and A6 were extracted with sodium salicylate and the chemical and antigenic composition of both extracts determined. The extracts were concentrated by ultrafiltration and the serotype antigen, measured by the indirect hemagglutination test, was estimated to have a MW between 100,000 and 300,000. The chemical composition of sodium salicylate extracts (SSE) from the 2 serotypes was similar, having protein, carbohydrate, fatty acid and P present in the ratio 10:1:0.5:0.1. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the SSE gave similar profiles with at least 48 bands present. Evidently, sodium salicylate removes the outer membrane of P. haemolytica. Crossed immunoelectrophoresis indicated that a major serotype-specific antigen was present in SSE of both strains. This antigen was extracted from the SSE with hot phenol/H2O and analyzed by gas chromatography. The sugar composition of A1 and A6 phenol/H2O extract (PWE) was qualitatively identical; some differences in proportions were observed. A1 and A6 PWE antigens produced mice against homologous serotype challenge and A6 PWE protected against heterologous (A1) challenge.