Plasma is first separated from the blood by centrifugation at 3300 rpm for 20 minutes. Mononuclear cells are then isolated by Ficoll-Hypaque density gradient centrifugation and deoxyribonucleoside triphosphates are extracted with 0.4 NHClO4, and the acid-insoluble material is removed by centrifugation. The supernatant is then carefully neutralized to pH 7 with potassium hydroxide, and the precipitated KClO4 is then removed by centrifugation. An ion-exchange high performance liquid chromatography method is then used to separate and quantitate the dFdCTP.