Recombinant plasmids coding for a soluble (or surface) antigen ofPasteurella haemolytica Al were identified. Two plasmids, both containing the same 5.4 kilobase pairs of insert DNA, were recovered independently by screening a clone bank of P. haemolytica Al genomic DNA in Escherichia coli for the expression of P. haemolytica Al soluble antigens (R. Y. C. Lo and L. A. Cameron, Can. J. Biochem. Cell Biol. 64: [73][74][75][76] 1986). E. coli cells carrying the plasmids were found to be agglutinated by an antiserum raised against the P. haemolytica Al soluble antigens. Analysis of the E. coli clones by electron microscopy revealed patches of amorphous material on the surface of the cells which were not present on the controls. Further characterization with protein A-colloidal gold labeled both these patches and the outer membranes of these cloned cells pretreated with the specific antiserum. These results indicated that the cloned antigen was expressed on the surface of the E. coli cells. The cloned antigen was found to be specific for serotype 1 when tested by slide agglutination against a collection of P. haemolytica typing antisera. Southern blot hybridization, using the cloned DNA as a probe, labeled the genomic DNA from P. haemolytica serotype 1 as well as the cross-agglutinating serotypes 2 and 7, but not DNA from the non-cross-agglutinating serotypes 3 and 4 and PasteureUa multocida. These results demonstrated that serotype specificity could be attributed to the particular antigenic determinants in the genome of the organism.