36 Mice were immunized with two hundred microliters of each antigen-adjuvant mixture, containing 10 mg of F1. After 30 days, the animals were boosted with the identical dose at the same inoculation site. Four negative control groups, consisting of 10 mice each, were immunized with the adjuvants only. F1 antibody titers of all animal groups were measured 26 days after the second F1-immunizing dose [Ref84:Andrews et al., 1996]. 36 Rats were vaccinated with F1 antigen in Freund's complete adjuvant. The rats received an injection of 500 ug of F1 followed by booster injections of 200 ug of F1 at 7 and 14 days. These rats were challenged 6 weeks later with 3.5 * 10^3 Y. pestis 195/P[Ref105:Williams et al., 1979] . Exposure to F1, either through vaccination or infection, stimulates the production of antibodies that can be measured quantitatively. At present, the passive haemagglutination (PHA) and haemagglutination-inhibition techniques are most widely employed for this purpose because the procedures are convenient and exhibit great sensitivity and specificity. Although the occurrence of antibody to F1 in man or animals suggests that some degree of protection against reinfection has been acquired, the relationship between the serological titre of F1 antibody and immunity to plague has not been clearly defined. The work reported here investigated the correlation between titre and protection with reference to the PHA procedure for measuring F1 antibody [Ref105:Williams et al., 1979].