The survival of 7 of 8 patients with Ebola virus (EBOV) infection after transfusions of convalescent-phase blood during a 1995 outbreak of EBOV infection is frequently cited as evidence that passive immunotherapy is a viable treatment option. To test whether whole-blood transfusions were more efficacious than passively administered immunoglobulins or monoclonal antibodies, we transfused convalescent-phase blood from EBOV-immune monkeys into naive animals shortly after challenge with EBOV. Although passively acquired antibody titers comparable to those associated with effective vaccination were obtained, all monkeys that had received transfusions succumbed to infection concurrently with control monkeys. These data cast further doubt on the value of passive immunotherapy for the treatment of EBOV infection.