Processing of a protein antigen into fragments is believed to be a prerequisite for its presentation by the antigen-presenting cell to the T cell. This model would predict that, in oligomeric proteins, T cells prepared with specificity for regions that are buried within subunit asiation surfaces should recognize the respective regions in vitro equally well on the isolated subunit or on the oligomer. Three hemoglobin (Hb) a-chain synthetic peptides, corresponding to areas that are situated either completely [a-(31-45)] or partially [a-(41-45) and a-(81-95)] within the interface between the a and fi subunits of Hb, and a fourth peptide representing a completely exposed area in tetrameric Hb were used as immumogens in SJL/J (H-2') mice. Peptide-primed T cells were passaged in vitro with the respective peptide to obtain peptide-specific T-lymphocyte lines. T-cell clones were isolated from these lines by limiting dilution. T-cell lines and clones that were specific for buried regions in the subunit association surfaces recognized the free peptide and the isolated subunit but not the Hb tetramer. On the other hand, T cells with specificity against regions that are not involved in subunit interaction and are completely exposed in the tetramer recognized the peptide, the isolated subunit, and the oligomeric protein equally well. The responses of the T-cell lines and clones were major histocompatibility complex-restricted. Since the same x-irradiated antigen-presenting cells were employed, the results could not be attributed to differences or defects in Hb processing. The findings indicate that in vitro the native (unprocessed and undissociated) oligomeric protein was the trigger of major histocompatibility complex-restricted T-cell responses.The presentation of a protein antigen to T lymphocytes is believed to be dependent on a first step in which the protein is internalized and processed into fragments that reappear on the surface of the antigen-presenting cell (APC) and are presented in association with major histocompatibility complex (MHC) molecules to the T cell (1, 2). Although there have been reports that a protein molecule is presented intact by the APC (see Discussion), the idea that processing is a prerequisite for presentation is by far the most widely accepted model. Many consequences of this model can be predicted and tested. If protein fragments and not the intact protein are presented by the APC to the T cell, it would be expected that T cells that are specific for the subunit interface in an oligomeric protein (if such T cells could be made) should recognize the isolated subunit or the oligomer equally well. Obviously, the T-cell recognition site cannot remain buried (in the interface between the subunits) after the oligomer is processed into fragments. If, on the other hand, these T cells recognize the peptide and the isolated subunit but not the oligomer, then this would be indicative that the subunit interface has remained buried in an intact oligomeric protein.In this paper we report the ro...