The role of anti-viral antibodies in homologous protective immunity to a virulent African swine fever virus (ASFV) strain E75 was examined by passive transfer experiments in swine. Eighty-five percent of animals (n = 14) that received anti-ASFV immunoglobulin (Ig) survived challenge infection, while 100% mortality was observed in control group animals (n = 28) that received anti-pseudorabies virus Ig, normal swine Ig, or phosphate-buffered saline. With the exception of a significantly delayed and transient fever response, anti-ASFV Ig group animals remained clinically normal following challenge, whereas control group animals presented with clinical ASF on Day 4 postchallenge. Additionally a significant 3 day delay in onset of viremia and a 10,000-fold reduction in both mean and maximum virus titers were observed for animals given anti-ASFV Ig. These results indicate that anti-ASFV Ig alone will protect swine from lethal infection with virulent ASFV. Further, they support the view that the antibody-mediated protective effect is an early event that effectively delays disease onset.