African swine fever virus contains nucleoside triphosphate phosphohydrolase activity which releases 32P phosphate from gamma-32P ATP at a rate of about 1 mumol/h mg of virus protein. The hydrolase activity is slightly stimulated by adding nucleic acids to the reaction mixture and under conditions of RNA synthesis. A study of the rate of ATP hydrolysis at different concentrations of ATP suggests the existence of two phosphohydrolase activities with apparent Km values of about 0.04 and 1 mM.