After they have been transfected with the herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2) thymidine kinase (TK) gene murine mammary carcinoma (FM3A) cells become highly sensitive to the growth inhibitory properties of the antiherpetic agents (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), 9(-)[(2-hydroxyethoxy)methyl]guanine (acyclovir, ACV), 9(-)[(1,3-dihydroxy-2-propoxy)methyl]guanine (DHPG, ganciclovir), and 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-methyluracil (FMAU). BVDU was 100-fold more potent an inhibitor of HSV TK gene-transfected tumor cell growth (50% inhibitory concentration (IC50), 0.0020-0.0047 microM) than FMAU or DHPG (IC50, 0.051-0.277 microM) and 1000-fold more potent than ACV (IC50, 0.42-4.9 microM). As a rule, the test compounds were more cytostatic to HSV-2 TK than HSV-1 TK gene-transfected FM3A cells. This may be ascribed to the higher phosphorylating capacity (Vmax/Km) of HSV-2 TK than HSV-1 TK and/or to the higher TK enzyme levels of the HSV-2 TK gene-transfected FM3A cells than the HSV-1 TK gene-transfected FM3A cells. Thymidylate synthase of the HSV TK gene-transfected FM3A cells appears to be the target enzyme for the cytostatic action of BVDU, but not FMAU, DHPG, or ACV. Instead, the cytostatic activity of DHPG seems to be correlated with its conversion to the triphosphate form and subsequent incorporation into the DNA of HSV TK gene-transfected FM3A cells.