Abstract: Methods for determining MHC class II binding activity of a preparation comprising lymphocyte activation gene-3 (LAG-3) protein, or a fragment, derivative, or analogue thereof, is described. The methods comprise determining binding of the LAG-3 protein, fragment, derivative, or analogue to MHC class II molecules using bio-layer interferometry (BLI). Such methods can be used as a quality control assay in good manufacturing practice (GMP) grade production of such compounds. Probes and kits for carrying out the methods are also described.