A single, subcutaneous, 30-microg dose of either a combination of the Yersinia pestis proteins F1+V or a F1-V fusion protein adsorbed to the adjuvant aluminum hydroxide, protected Hsd:ND4 mice for one year against pneumonic plague. The recombinant F1+V vaccine provided significant protection as early as day 14 postimmunization. The current Plague Vaccine USP in a single 0.2-ml dose did not provide significant protection in this mouse model. Antibody titers to F1 and V peaked at approximately 5-12 weeks postimmunization and were still detectable one year later. These F1 and V subunit vaccines may offer effective long-term immunity with a reduced dosage schedule when compared with the presently licensed, formalin-killed, whole-cell vaccine.