SUMMARYThe objective of this study was to identify an immunological correlate of protection for a twocomponent subunit vaccine for plague, using a mouse model. The components of the vaccine are the F1 and V antigens of the plague-causing organism, Yersinia pestis, which are coadsorbed to alhydrogel and administered intramuscularly. The optimum molar ratio of the subunits was determined by keeping the dose-level of either subunit constant whilst varying the other and observing the effect on specific antibody titre. A two-fold molar excess of F1 to V, achieved by immunizing with 10 mg of each antigen, resulted in optimum antibody titres. The dose of vaccine required to protect against an upper and lower subcutaneous challenge with Y. pestis was determined by administering doses in the range 10 mg F1 þ 10 mg V to 0·01 mg F1 þ 0·01 mg V in a two-dose regimen. For animals immunized at the 1-mg dose level or higher with F1 þ V, an increase in specific IgG1 titre was observed over the 8 months post-boost and they were fully protected against a subcutaneous challenge with 10 5 colony-forming units (CFU) virulent Y. pestis at this time point. However, immunization with 5 mg or more of each subunit was required to achieve protection against challenge with 10 7 CFU Y. pestis. A new finding of this study is that the combined titre of the IgG1 subclass, developed to F1 plus V, correlated significantly (P < 0·05) with protection. The titres of IgG1 in vaccinated mice which correlated with 90%, 50% and 10% protection have been determined and provide a useful model to predict vaccine efficacy in man.