From a KOS herpes simplex virus strain prepared in VERO cells we have been able to obtain after several exposures to increasing concentrations of trifluorothymidine, four resistant mutants. Among these strains, 31 T mutant was a TK-altered strain of low pathogenicity, sensible to acyclovir and did not induce recurrences. These properties might allow the use of such a strain as a living attenuated antiherpetic vaccine.