Abstract. Irradiation of legume seeds has emerged as an attractive alternative compared to conventional chemical treatments in seed production. Irradiation is also used for the decontamination of food and feed in order to increase the shelf-life of fresh and dry food materials. The effects of irradiation on nutritive and anti-nutritive factors such as trypsin inhibitors are usually reported together with the measurements obtained by using the quantitative analytical methods. The objective of this study was to measure trypsin inhibitor activity (TIA) of common bean cultivar Oplenac using the microtiter plate method and to identify factors that contribute to the uncertainty of TIA measurement according to the current Guide to the Expression of uncertainty in measurement (GUM). Dominant sources of uncertainty of TIA measurement were: absorbance measurements of sample and positive control reaction mixtures and preparation of the final sample solution using a graduated cylinder (V4). Absorbance measurement of sample reaction mixtures took 37.8 % of the overall measurement uncertainty. Preparation of the final sample solution using a graduated cylinder (V4) and absorbance measurement of positive control reaction mixture contributed to the overall uncertainty with 35.1 % and 15.8 %, respectively. Acquired insight into factors that contribute to the uncertainty of TIA measurement gives directions for the improvement of TIA testing methods and TIA results management.