Human lymphocyte kynureninase activity was assessed in homogenized cells by determination of 3-hydroxyanthranilic acid production as a function of time after addition of the substrate, 3-hydroxykynurenine. The product, 3-hydroxyanthranilic acid, was determined by isocratic high-performance liquid chromatography and fluorescence detection. Mean (+/- S.D.) lymphocyte kynureninase activity in a group (n = 12) of vitamin B6-deficient men was 5.04 +/- 0.81 pmol 3-hydroxyanthranilic acid formed per mg protein per min, which was significantly (p = 0.005) lower than the 6.69 +/- 1.70 pmol 3-hydroxyanthranilic acid formed per mg protein per min in men with a normal vitamin B6 status. This indicates that lymphocyte kynureninase activity is depressed during a vitamin B6 deficiency.