This unit describes a technique for the direct and quantitative measurement of the capacity of peptide ligands to bind Class I and Class II MHC molecules. The binding of a peptide of interest to MHC is assessed based on its ability to inhibit the binding of a radiolabeled probe peptide to MHC molecules. The establishment of an MHC/peptide binding assay, and its subsequent use in determining the MHC binding capacities of peptide ligands, requires sufficient stocks of purified MHC and both labeled and unlabeled peptides. Accordingly, this unit includes protocols for the purification of Class I and Class II MHC molecules by affinity chromatography, and for the radiolabeling of peptides using the chloramine T method. A support protocol describes alterations in the basic protocol that are necessary when performing direct binding assays, which are required for (1) selecting appropriate high-affinity, assay-specific, radiolabeled ligands and (2) determining the amount of MHC necessary to yield assays with the highest sensitivity. After a 2-day incubation, the bound and unbound radiolabeled species are separated, and their relative amounts are determined. Two methods for separation by size-exclusion gel-filtration chromatography are described, as is data analysis.