Sensitive and selective high-performance liquid chromatographic techniques have been developed for the determination of 2'-3'-didehydro-3'-deoxythymidine, d4T (BMY-27857), in human plasma and urine. The methods had linear standard curves over the concentration ranges 0.025-25.0 and 0.5-100 micrograms/ml for the plasma and urine matrices, respectively. Both methods used solid-phase extraction for isolating d4T and the internal standard, thymidine oxetane, from the biological matrix. In addition, the analytical column, mobile phase, instrumentation and chromatographic conditions used for both methods were identical. The ultraviolet absorbance of the column effluent was monitored at 266 nm. Results of analysis of quality control samples indicated that the intra-assay precision values, as measured by percent relative standard deviation, were within 12 and 3%, and accuracy samples deviated less than 10 and 5% from nominal values for the plasma and urine assays, respectively.