A rapid, sensitive and reliable reversed-phase high-performance liquid chromatographic (HPLC) method with UV detection has been developed for the assay of a novel anti-herpes agent, 9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine (BRL-39123), in human plasma and urine. The drug and the internal standard, the structural analogue BRL-42377, were extracted from the biological matrix by adsorption on a cation-exchange column and were subsequently eluted under alkaline conditions prior to HPLC. The method is reproducible, with coefficients of variation of ca. 5%, and linear from 0.1 to at least 30 micrograms ml-1 in plasma and from 50 to at least 2000 micrograms ml-1 in urine. The method has been used extensively to measure BRL-39123 in plasma and urine samples generated during clinical studies and is adequate for defining pharmacokinetics at projected therapeutic doses.