T cells recognize foreign antigens in association with the highly polymorphic class I and class II molecules encoded in the major histocompatibility complex (MHC). In addition to these highly polymorphic molecules, the murine MHC also encodes, in the Qa/Tla region, several less polymorphic structures referred to as class I-like or class Ib molecules. Although no specific function has been assigned to these molecules, their overall structural similarities to the classical class I molecules and their association with beta 2-microglobulin suggest a role in antigen recognition. Recent data have suggested that the class Ib molecule Qa-1 may be involved in antigen presentation to T cells expressing gamma delta receptors. In addition, several reports have demonstrated that gamma delta T cells can respond to mycobacterial heat shock proteins. We report that transfection of a mouse fibroblast line with gene T23b leads to the surface expression of a molecule that is structurally identical to lymphocyte Qa-1b. In the transfected cells the predominant Qa-1 species was present in an immature intracellular form. The expression of mature cell surface Qa-1 was dramatically and selectively increased following heat shock. Furthermore, the addition of a tryptic digest of Mycobacterium bovis 65-kDa heat shock protein stabilized the surface expression of Qa-1b. These observations suggest that the Qa-1 molecule may be involved in the presentation of heat shock protein-derived peptides to the immune system.