The function of MHC class-I molecules is to sample peptides from the intracellular environment and present them to CD8+ cytotoxic T lymphocytes. To understand the molecular details of the assembly (and disassembly) of peptide-beta 2m-class-I complexes a biochemical peptide-class-I binding assay has been generated recently and this paper reports on a similar assay for the interaction between beta 2m and class I. As a model system human beta 2m binding to mouse class I was used. The assay is strictly biochemical using purified reagents which interact in solution and complex formation is determined by size separation. It is specific and highly sensitive. The observed affinity of the interaction, KD, is close to 0.4 nM. The rate of association at 37 degrees C is very fast (the ka is around 5 x 10(4)/M/s) whereas the dissociation is slow (the kd is around 8 x 10(-6)/s); the ratio of dissociation to association yields a calculated KD close to the observed value. At 37 degrees C almost all of the purified class I participates in binding of the exogenously offered beta 2m showing that a considerable exchange of the endogenous beta 2m occurs. Finally, it was demonstrated that exogenous beta 2m enhances binding to MHC class-I of short perfectly-matching peptides as well as longer peptides.