An African swine fever virus-induced protein of molar mass 12 kDa (p12) was studied in virus-infected Vero cells using the monoclonal antibody 18B.B11. Protein p12 is incorporated into the membrane of infected cells about 7 h post-infection and is not present in purified African swine fever virus particles. The synthesis of protein p12 is sensitive to cytosine arabinoside.