The cytotoxicity and the antivirus activity of native hemocyanin, RtH, derived from the Bulgarian marine mollusk Rapana thomasiana and its structural isoform, RtH2, against HSV replication was evaluated on three HSV strains Ð two wt strains, TM (HSV 1) and Bja (HSV 2), and one ACV R mutant with tk gene mutation, DD (HSV 2). The experiments were performed on continuous RD 64 cells and three HSV 1 and HSV 2 strains were used, two mutants sensitive to acyclovir and one resistant mutant.Both compounds were found to be effective inhibitors of wt HSV replication. Both compounds did not exhibit any effect on the infectious virus yield on ACV R mutant. The most promising, active and selective, anti-HSV agent, especially to genital herpes virus, was found to be the functional unit of native hemocyanin Ð RtH2. RtH2 did not induce apoptosis/ necrosis 8 h after virus infection and the target of its action, was found to be the viral but not the host cell DNA.