An assay method for the determination of acyclovir from pharmaceutical preparations has been developed for assessment of product quality utilising high-performance liquid chromatography. The chromatographic conditions comprised a reversed-phase C18 column (250 x 4.6 mm i.d.) with a mobile phase of acetonitrile-20 mmol l(-1) aqueous ammonium acetate buffer of pH 4.5 (40:60). The flow rate was 0.8 ml min(-1) and UV detection was used at 250 nm. Calibration graph was linear in the range 1.98-59.4 microg ml(-1). The method has been validated according to current guidelines including assay of pharmacopoeial standard tablets. Recoveries ranged from 96.64 to 99.53%. The exipients present in the tablets did not interfere with the method.