Monoclonal antibody-reactive antigenic regions on the 99 amino acid sequence of human beta 2-microglobulin were examined by using nine anti-beta 2 m monoclonal antibodies (mAbs) and Geysen pins coated with overlapping heptamers derived from the primary sequence. Reactive linear sequences were then restudied with glycine substitution for each successive residue. All nine mAbs showed a similar profile, with peak reactivity at residues 57-63 SKDWSFY and 87-97 LSQPKIVKWDR. Glycine substitution established the tryptophans at positions 60 and 95 as well as the lysines at 58, 91, and 94 as immunodominant residues important for mAb reactivity. Other adjacent residues (serine 57, phenylalanine 62, tyrosine 63, isoleucine 92, valine 93, and proline 90) also make important contributions to beta 2 m antigenic epitopes. Rabbit antipeptide antibodies against the beta 2 m peptides SKDWSFY and LSQPKIVKWDR produced quantitative precipitin curves with beta 2 m in solution. Addition of iodine 125-labeled beta 2 m to antipeptide antibody followed by protein A precipitation and sodium dodecyl sulfate gel electrophoresis with autoradiography showed that labeled beta 2 m reacted in solution phase with anti-SKDWSFY or anti-LSQPKIVKWDR antibody. These findings establish residues 57-63 and 87-97 in human beta 2 m as major reactive antigenic sites for monoclonal as well as polyclonal anti-beta 2 m antibodies.