Although T cells expressing the gamma delta Ag receptor (gamma delta cells) have been found in all organisms with lymphoid systems, little is understood about their immunologic function. It is known that T cells that express the alpha beta Ag receptor (alpha beta cells) fight infection by recognizing foreign Ag as peptide presented by MHC molecules; however, it is not known how gamma delta cells recognize Ag. Characterization of ligands of the gamma delta cell Ag receptor (TCR) could lead to a better understanding of gamma delta cell function. We have characterized the ligand of the G8 gamma delta cell; we find that G8 gamma delta cells are stimulated by T22, a nonclassical class I MHC molecule. To study the involvement of peptides in gamma delta cell recognition, T22 was expressed in cells deficient in peptide transporter function and in Drosophila cells, which can express MHC molecules without associated peptides. We find no evidence that G8 gamma delta cells recognize peptides bound to T22. These results suggest that some gamma delta cells, unlike alpha beta cells, may not be restricted to recognition of peptide-MHC complexes.