Antibody neutralization of African swine fever (ASF) virus measured by a plaque reduction assay presents frequent difficulties because of the absence or delay in plaque formation by many strains, especially low-passage viruses. To overcome this problem, a new ASF virus neutralization test has been developed. The new test consists of a conventional plaque reduction assay in which the viral plaques are detected by expression of marker genes. For the development of this neutralization assay 4 mutant viruses were generated by homologous recombination, containing beta-galactosidase or beta-glucuronidase reporter genes inserted into the thymidine kinase locus of the viral genome. These recombinant viruses have the following advantages with respect to parental viruses: (1) the neutralization assay takes less than a third of the time needed using non-recombinant viruses; (2) the small plaques can be detected more accurately by color contrast; and (3) the neutralization-resistant virus clones can be recovered easily post-plaque counting. Additionally, these recombinant viruses permit differentiation by chromogenic staining of individual infected pig macrophages, the natural host cell for ASF virus, facilitating neutralization assays in these primary cultures as described in cell lines.