The inhibitory potency of new analogs of nucleoside 5'-triphosphates modified at the sugar residue and or alpha-phosphate against herpes simplex virus type 1 DNA polymerase has been evaluated in a cell-free system containing M13mp10 phage DNA and a synthetic primer. Triphosphates of new acyclic nucleosides [1-(5-hydroxy-2-cis-pentenyl)nucleosides] were the most effective inhibitors among 15 types of nucleoside 5'-triphosphates under investigation, being threefold less active than acyclovirtriphosphate. 5'-Phosphonylmethyl-2'-deoxythymidine beta, gamma-diphosphate proved to be a poor substrate for DNA polymerase. Compounds with other modifications at alpha-phosphate were inactive. Constants of hydrolysis rate of acyclonucleosides incorporated into the 3' end of primer were determined.