Proteins are biological macromolecules composed of amino acids linked by peptide bonds. Not only are they the main metabolic products of living organisms, but many of them can be used for therapeutic or diagnostic purposes and in many other areas such as the cosmetics and food industries. In order to use them safely, it is important to determine their concentrations accurately. In current practice, various methods are used to determine their concentrations. However, the most commonly used are the Biuret assay, the Lowry assay, and the Bradford assay, as they are rapid, easy to use, and inexpensive. The major drawback of the aforementioned assays is that the accuracy of the measurement can be affected by other molecules present in the protein solution. The aim of this study is to test and compare the sensitivity of the Biuret assay, the Lowry assay and the Bradford assay for the determination of a low protein concentration and the sensitivity of each assay to the presence of different molecules in the protein solution. This study investigates the influence of some compounds (salts, metals, sugars, detergents, organic solutions, acids and bases, DMSO and EDTA) on the accuracy of the Biuret assay, the Lowry assay and the Bradford assay. Two different concentrations of a standard solution of bovine serum albumin (BSA) (γBSA = 5 i 50 mg/L ) were prepared, with which all assays were performed. The results obtained showed that a large number of compounds tested reduced the accuracy of the chosen methods. At the very end, the protein concentration in the crude extract of lipase and xylanase enzymes (without additional purification steps) produced by fermentation of Thermomyces lanuginosus on pumpkin cake was determined.