Antibody-containing plasma from patients recovered from Argentine hemorrhagic fever (AHF) is of proven value in treatment of the acute disease, but the possibility of transmitting blood-borne organisms such as HIV and hepatitis virus detracts from this approach. Purified human immune plasma fractions IgG1,2,4, IgG1,2,3,4 and F(ab')2 neutralized Junin virus in vitro. IgG1,2,3,4 and IgG1,2,4 lysed (in the presence of complement) cells infected with Junin virus, and protected infected guinea pigs from AHF. However, large quantities of the immune F(ab')2 fraction from the same plasma pool failed to protect guinea pigs from death, to increase the mean time to death, and to diminish virus load in target organs of infected guinea pigs. This suggests that elimination of infected cells rather than virus neutralization may play a critical role in protection against Junin virus.