The structural requirements for signal transduction by class I major histocompatibility complex (MHC) molecules were examined. Native or mutant HLA-A2 or HLA-B27 constructs lacking most of their cytoplasmic domains were co-transfected with pSV2neo into Jurkat cells. Transfection of either native or mutant constructs resulted in a comparable expression of the gene products. Stimulation of transfectants expressing either native or truncated A2 or B27 molecules with specific mAb evoked an increase in [Ca2+]i upon crosslinking. Moreover, crosslinking native or truncated A2 or B27 induced IL-2 production upon co-stimulation with phorbol myristate acetate. These results confirm that crosslinking class I MHC molecules transduces an activation signal to human T cells. Effective signaling was observed when all but four of the intracytoplasmic residues were deleted, indicating that signal transduction does not require this portion of the molecule.