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Although strain 19 vaccine has been widely recognized as the most effective vaccine for the control of bovine brucellosis, its use in the United States has declined in the last ten years. When the national level of infection was reduced to 1%, it was believed possible to move towards complete eradication of bovine brucellosis. The continued use of strain 19 vaccine was thought to interfere with the goal of complete eradication because: 1. persistence of serum agglutination titers in some animals caused confusion in the serological diagnosis, 2. occasional persistence of strain 19 infection in adult animals caused reactions in surveillance tests which required follow-up herd tests, 3. vaccination was reported to "mask" virulent infection in a herd by prolonging the incubation period. These perceived disadvantages of strain 19 are discussed in relation to the benefits derived from its use in infected areas. In the last ten years the number of calves vaccinated with strain 19 has steadily declined, whereas the national average of infected cattle has remained stationary. The level of infection in some areas has increased however. The date for complete eradication of bovine brucellosis in the United States has had to be postponed. A retrospective examination of the brucellosis eradication program shows that in general those states which vaccinated 40% or more of their eligible calves with strain 19 were able to eradicate brucellosis. States which still have a bovine brucellosis problem, in general, have never employed wide-scale vaccination. Other factors contributing to these differences in the progress of eradication will be discussed. The important role of strain 19 in the successful eradication of bovine brucellosis in some areas of the United States has been the reduction in the level of infection to the point where eradication was economically feasible.

Brucella endotoxin differs from other gramnegative endotoxins in that it is recovered in the phenol phase rather than the aqueous phase of the Westphal hot phenol water procedure. This was the first described from this laboratory by Redfearn (1960) with phenol-killed smooth B. abortus and B. melitensis and has since been confirmed by others. Preliminary extraction of brucella cells with acetone, as called for in the original Westphal procedure, was followed by Renoux et al. (1973) who reported that the aqueous phase lacked endotoxic activity and the phenol phase had very low toxicity. In order to test the hypothesis that prior acetone extraction removes lipid A, we have repeated the Redfearn procedure with acetone extracted cells and have confirmed that the major portion of the endotoxic activity resides in the phenol phase. Acetone treatment does not remove the lipid A believed to be responsible for mouse lethality as well as necrotizing activity in guinea pig and rabbit skin. Preparations of brucella endotoxin (lipopolysaccharide) contain varying amounts of polypeptide some of which is tightly bound. The dermal response of sensitized guinea pigs to brucella LPS was shown to be a combination of reactions comprising those due to (1) innate toxicity of lipid A, (2) antibody mediated reactions due to polysaccharide portion of the molecule and (3) delayed hypersensitivity due to polypeptide portion of the molecule.

In the development of a biological potency assay for brucella allergens by skin testing of guinea pigs, it was observed that the delayed hypersensitivity reactions to a given dose depended upon (1) the method of sensitization of the guinea pigs, (2) the time interval between sensitization and skin test and (3) the total of allergen given per animal. Data illustrating these factors are presented and a method for titrating allergens is recommended.