Feline immunodeficiency virus (FIV) causes progressive impairment of the feline immune system, in time leading to acquired immunodeficiency syndrome. A hallmark of FIV infection is the depletion of CD4+ T cells and a reduction in the CD4:CD8 lymphocyte ratio. This study’s purpose was to evaluate whether a 1-year duration of immunity against sub-type B FIV is attainable with the Fel-O-Vax FIV vaccine. Fourteen vaccinated cats were challenged with a subtype-B FIV strain 54 weeks after final vaccination. Ten of the fourteen vaccinated cats were fully protected for 48 weeks against infection with this FIV subtype. The levels of anti-FIV antibody noted for the vaccinated cats suggests that the three-dose vaccination regimen is capable of establishing and maintaining immunologic memory for at least 12 months. [VT]