Methods for the modification of ganciclovir (GCV), acyclovir (ACV), cidofovir (CDV) and valganciclovir (VCDV) with boron cluster have been developed. Toxicity of the new derivatives was evaluated in adherent cells; no cytotoxicity was observed in five different cell lines up to 1000 microM with the exception of modified valganciclovir which was cytotoxic above 300 microM. The compounds were active against HCMV or HSV-1 by cytopathic effect or plaque reduction assays. None of the tested compounds had activity against HPIV-3 or VSV.