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The degree of agglutination in the tube agglutination test of the sera of 373 "reactor" cattle was compared to that of 800 "non-reactor" cattle from multiple reactor herds in southwestern Ontario. Vaccinated cattle had lower titers than did nonvaccinated cattle. Thus, vaccinated cattle were less likely to be classified as positive than nonvaccinated cattle irrespective of the method of interpreting the results of the tube test. When the full allowance for vaccination was used, vaccinated cattle were less likely to be classified as suspicious than nonvaccinated cattle. The extent of herd vaccination did not appear to markedly influence the effects of vaccination or the prevalence of reactors. Thus, it appeared that vaccination with strain 19 produced a small protective effect (inferred from the lower titers in vaccinated animals), but that the magnitude of this effect was markedly dependent on the appropriateness of the vaccine allowance for classifying cattle. Further work is needed on this aspect of vaccination with strain 19.