A simple, rapid, and accurate high-pressure liquid chromatographic method with fluorescence detection is described for the measurement of tafenoquine (TQ) (also known as WR 238605) from human plasma and venous and capillary blood. Tafenoquine was measured in plasma and venous blood following protein precipitation. Chromatographic separation was achieved using a Waters S5P Spherisorb phenyl analytical cartridge (150 mm x 4.6 mm I.D., 5 microm particle size) (Waters, Milford, MA, USA) and a mobile phase of 22 mM ammonium acetate, pH 4:acetonitrile (45:55, vol/vol). The flow rate was 1.5 mL/min and the retention times were approximately 3.5 min for WR VIIIAc (internal standard) and approximately 7.8 min for TQ. The interday and intraday coefficients of variation of TQ over a concentration range of 20-1000 ng/mL in plasma were < or =8.4% and in venous blood were < or =9.6%. The mean percent difference between added concentration and obtained concentration was 7.3% in plasma and 8.5% in venous blood over the corresponding concentration range. The limit of quantitation for both fluids was 10 ng/mL. Tafenoquine concentrations were comparable between capillary and venous blood with no significant difference between measurement in both biological fluids. The clinical application of the method was demonstrated by measuring plasma and whole blood concentrations of TQ from participants in a chemosuppression trial of the drug against malaria infections in Thailand.