The authors performed a detailed comparative clinical and pharmacokinetic study with two 800 mg acyclovir containing tablets, namely the Telviran (EGIS Pharmaceuticals Ltd.) and the Zovirax (Wellcome Foundation Ltd.). The determination of the detailed pharmacokinetic parameters and the relative bioavailability was carried out on 24 healthy male volunteers in a two way, open, randomised, cross-over design study after single dose administration. The plasma concentration of acyclovir was determined by a newly developed and validated highly sensitive (LLOQ = 10 ng/ml) HPLC-UV bioanalytical method after sample preparation with RP-18 solid phase extraction method (SPE). The individual pharmacokinetic parameters calculated from the time-plasma concentration curve (tmax, Cmax, AUC0-infinity, AUC0-16, AUC0-t, AUC0-z, AUCRest, t beta 1/2, MRT, Cmax/AUC0-infinity) were compared with statistical methods (ANOVA, ANOVAlog, Confidence interval, Schuirmann, Wilcoxon tests). On the basis of the results of the statistical evaluation and the clinical study, there was no significant difference found between the two acyclovir containing preparations. The comparative pharmacokinetic study demonstrated, that the relative bioavailability of the 800 mg Telviran and Zovirax tablets are equivalent and the two products are bioequivalent.