A persistent non-neutralized African swine fever virus (ASFV) fraction is found with most convalescent swine sera in in vitro neutralization assays. To study this phenomenon, antisera from convalescent pigs infected with different virus isolates and showing complete or incomplete virus neutralization were used. Different experiments determined that incomplete neutralization of ASFV is caused neither by virus aggregation, nor low affinity or stability of virus-antibody complexes. Additionally, attempts to purify antigenic escape mutant viruses from the persistent fraction was also unsuccessful. Nevertheless, competition experiments between sera demonstrated that antibodies present in sera showing persistent fraction inhibited the complete neutralization mediated by antibodies present in sera which neutralize 100% of virus infectivity. These results suggest that induction of blocking antibodies during ASFV infection could represent the main cause for the persistent surviving virus fraction observed in neutralization assays and could also explain the persistent infections observed in some convalescent pigs.