β-(1→3)-d-Glucan is a branched polysaccharide present in the inner layer of the cell wall of various representatives of the fungal kingdom. Therefore, this polysaccharide is considered as a common marker for the laboratory diagnosis of mycoses of various etiologies. However, low concentration of β-(1→3)-d-glucan in biological fluids (∼40 pg mL⁻¹) requires the development of highly sensitive methods for its determination. One of the most convenient types of bioassays is based on immunochemical approaches, in which the most important stage is selection and characterization of monoclonal antibodies (mAb). In this communication, the binding constant of mAb 3G11 and the synthetic oligosaccharide β-d-Glcp-[(1→3)-Glcp]₈, which is related to the linear moiety of fungal glucan, was determined in solution for the first time using fluorescence polarization immunoassay (FPIA) method. The obtained information will be used subsequently for the development of analytical systems for FPIA detection of β-(1→3)-d-glucan.