Concentration dependency of formation of triphosphate esters of nucleoside analogs in virus-infected MRC-5 cells. MRC-5 cells infected with HSV-1 (a) or HSV-2 (b) were treated with various concentrations of [3H]A-5021, 3H[ACV], or [3H]PCV for 6 h. Then cells were extracted, and the extracts were analyzed by HPLC. Open circles, A-5021 triphosphate; closed triangles, ACV triphosphate; closed squares, PCV triphosphate. Conc, concentration.