A short review on available toxicological data of acyclovir is given. In principle, a substance which interferes with the nucleic acid metabolism should be judged with special care for carcinogenic and mutagenic properties. Although the substance is mainly incorporated into the viral genome, very high doses have also shown genotoxic effects in mammalian cell systems. However, since such effects are found only in doses where also naturally occurring nucleosides show the same effect, this manifestation does not appear to represent a potential hazard. In general, acyclovir seems to have a low toxic potential. Data from available investigations do not give support for a mutagen, teratogen or carcinogen hazard in patients receiving recommended clinical doses. An awareness of a potential risk for the fetus, especially with infusion treatment should nonetheless be kept in mind. The possibility of kidney damage with accumulation of the drug should also be taken into account. Such damage seems to have less importance with p.o. administration, and has little or no importance with the proposed use of eye ointment.