Tacaribe virus (TV) is the prototype of the Tacaribe group of serologically defined viruses geographically distributed in the Americas. Two of these viruses are human pathogens responsible for hemorrhagic fever, i.e., Junin virus in Argentina and Machupo virus in Bolivia. A particularly close antigenic relationship exists between TV and Junin virus manifested by their cross-reactivity at the level of antibody and by the ability of TV to protect animals from subsequent lethal challenges with Junin virus. TV, however, appears to be nonpathogenic for humans (reviewed by Howard, 1986).