The objective of the experiment was to fnd a quantitative test to determine the lysine or methionine bioavailability of rumen-protected amino acids. For these purposes we elaborated a blood test including an in vivo calibration phase and we used this test on two commercial products whose bioavailability is known (SmartamineM™ and SmartamineML™). Three ruminally and duodenally fistulated low-yielding (10 kg/d) Holstein cows were used. The calibration phase consisted of relating blood plasma methionine and lysine concentrations to duodenal infusion of graded amounts of methionine and lysine. The blood plasma responses of methionine and lysine to graded amounts infused duodenally were linear. The methionine response was the same for all cows, but the lysine response varied with the cows. Quantification of the bioavailability of commercial products is achieved by relating the blood level variations, as induced by product supplementation, to those obtained by duodenal infusion during the calibration phase. Bioavailability of methionine supplied by SmartamineM™ and SmartamineML™ was 75 and 84%, respectively. The reliability of the test is related to the amount of amino acid provided by the product supplementation: providing a low quantity of amino acids resulted in an overestimation of the bioavailability. This tendency is particularly clear for lysine bioavailability estimation. The proposed blood test is valuable for the determination of the bioavailability of methionine, but not lysine, of rumen-protected products.