A HPLC-UV method has been developed for assaying rifampicin in plasma and liver. The assay involved a liquid-liquid extraction procedure with dichloromethane-pentane (1:1). An Ultrabase-C18 column and a simple mobile phase consisting of a water (pH 2.27)-acetonitrile (40:60, v/v) mixture were used. The flow-rate was 1 ml/min and the effluent was monitored at 333 nm. Results from the HPLC analyses showed that the assay method is linear in the ranges 0.1-1 and 1-50 microg/ml for plasma, and 0.6-40 microg/g for liver. Intra- and inter-day R.S.D. were below 15% for all the sample types. Recoveries averaged 83 and 95% for plasma and liver, respectively. The method is being successfully applied to determine rifampicin in plasma and liver samples taken during pharmacokinetic studies in rats.