Transfer of anti-Sindbis virus serum, obtained from peripherally inoculated donors, protected mice from an otherwise fatal intracerebral infection with neuroadapted Sindbis virus (NSV). F(ab)'2 preparations of serum were not protective, indicating that the Fc piece of immunoglobulin G was important. Complement-depleted animals were protected with anti-NSV serum, ruling out as essential the complement-fixing function of the Fc piece. The presence of protective antibody correlated with the ability of serum to inhibit T-cell cytotoxicity. However, experiments using athymic nude mice showed that T cells played no role in killing the mice since the 50% lethal dose was the same as that in normal BALB/c mice, and that T cells were not required for protection since athymic nude mice were protected with antibody alone. Cyclophosphamide treatment of NSV-infected mice ablated the protective capacity of anti-NSV serum. Therefore, a non-T cell, cyclophosphamide-sensitive cell was required for antibody-mediated protection.