A high-performance liquid chromatographic method was developed for the determination of plasma purine nucleoside phosphorylase activity. In this method, the reaction mixture consisted of (15 \mu) l of plasma and (285 \mu \mathrm{l}) of (50 \mathrm{mM}) phosphate buffer ( (\mathrm{pH} 7.4) ) containing (3.8 \mathrm{~mm}) inosine and (0.15 \mathrm{mM}) 2-(3-cyano-4-isobutoxyphenyl)4-methyl-5-thiazolecarboxylic acid (strong xanthine oxidase inhibitor). After the reaction, the hypoxanthine produced was monitored to express plasma purine nucleoside phosphorylase activity. By this method, the activity of purine nucleoside phosphorylase was easily determined even with a small-volume plasma sample and despite its low activity in plasma. In addition, plasma purine nucleoside phosphorylase activity can be accurately determined even if the plasma is turbid. As a result, we were able to measure plasma purine nucleoside phosphorylase activity in patients with gout or asthma and healthy subjects, whereby it was demonstrated that plasma purine nucleoside phosphorylase activity was higher in patients with asthma than in either healthy subjects or patients with gout. (1995 \mathrm{Aca}-) demic Press, Inc.