In assay cultures, sera from African swine fever convalescent pigs inhibited infection by homologous African swine fever virus. The infection-inhibition capacity did not correspond with the virus-neutralizing capacity. The serum did not prevent infection by heterologous virulent viruses. Sera from pigs challenge inoculated with the homologous virulent virus and later with a heterologous virulent virus inhibited the infection by different heterologous virulent viruses. These sera did not interfere with the infection by pseudorabies virus. The specificity of the reaction indicated that the infection inhibition was caused by antibody.