A method to assess selenium status of the body by measuring glutathione peroxidase activity in erythrocytes was studied. Reaction was measured by continuous monitoring of the decrease of NADPH at 340 nm. The erythrocyte glutathione peroxidase activity was determined at 37 degrees C, pH 7.5 by using one of the substrates, t-butyl hydroperoxide, to initiate the reaction, and using glutathione reductase as the coupling enzyme. The Km of the enzyme for glutathione and t-butyl hydroperoxide were determined to be 1.8 mM and 238 microM, respectively. The enzyme was stable at -20 degrees C or -70 degrees C for at least 5 months, and for at least 2 months when stored at 4 degrees C. The within-run and between-run coefficients of variation for this method were 3.3-4.9% and 3.0-7.1%, respectively. The test was linear up to 100 U/g hemoglobin, and had a sensitivity of 0.002 delta A/min at 3 U/L. The reference range of erythrocyte glutathione peroxidase in Chinese adults was estimated to be 28.6-87.8 U/g hemoglobin (n = 84), without a significant difference in the results between males and females.