In this report we provide evidence for the expression of antigenic epitopes on mouse beta 2-microglobulin(b) (beta 2mb) that result from assembly with cognate H-2 class I heavy chains. For the cell line 69.9.15 (beta 2ma x beta 2mb), which expresses a mutant cytosolic form of H-2Kb and wild-type H-2Db, flow cytometry with rabbit antiserum against mouse beta 2m displayed beta 2m expression by cells grown in the presence or absence of fetal calf serum. By contrast, the epitopes identified by the beta 2mb-specific monoclonal antibody (mAb) S19.8 and clone 23 were not expressed by 69.9.15 cells grown in serum-containing conditions, and although S19.8 reactivity was weakly recovered by culture in the absence of serum, no such reactivity was observed with clone 23. Strong expression of these epitopes was achieved following transfection of 69.9.15 cells with the wild-type H-2Kb gene, indicating that the beta 2mb epitopes defined by mAb S19.8 and clone 23 were expressed when beta 2mb was assembled with an appropriate heavy chain. In support of this conclusion, we observed the recovery of the S19.8 and clone 23 epitopes by in vitro assembly of H-2Kb heavy chains with beta 2mb in the presence of the VSV N protein p52-59; however, such epitopes were expressed neither by beta 2mb prior to heterodimer assembly nor by non-conformed beta 2mb present in tissue culture supernatants recovered from H-2 class I surface positive cells. Taken together, these data indicate that in addition to the property of beta 2m to modify the antigenicity of the MHC class I heavy chains, beta 2m epitopes are induced in a reciprocal manner by assembly with MHC class I heavy chain molecules.