Major histocompatibility complex (MHC) class I molecules are cell-surface proteins that present peptides to CD8+ T cells. These peptides are mostly derived from endogenously synthesized protein. Recombinant, soluble MHC class I molecules were produced, purified, and loaded homogeneously with synthetic peptide. These MHC-peptide complexes were used to activate a T cell hybridoma. While monomers of MHC-peptide bound to the T cell, they showed no stimulatory activity. Dimers fully triggered the T cell hybridoma to secrete interleukin 2. This response was followed by a state in which the T cell was refractory to restimulation as a result of defective signal transduction through the T cell receptor.