Single amino acid substitutions of Ag and MHC were used to analyze the fine structure of the influenza hemagglutinin (HA)-derived epitope (HA 307-319) recognized in the context of DR7 molecules by a T cell clone. Putative T cell (HA 308, 310, 311, 313, and 316) and DR (HA 309, 312, and 317) contact residues of the Ag were identified by the use of single amino acid-substituted analogs that were tested for their T cell-activating and DR-binding capacities. The peptide-DR7-T cell interaction was further characterized by the use of a panel of 13 site-directed DR7 mutant transfectants analyzed for their capacity to present Ag to T cells, and for their purified mutant DR7 molecules to bind HA 307-319 or its single amino acid-substituted analogs. Eight mutants lost their Ag-presenting function, whereas only one had any decrease in peptide binding. Finally, for three of the mutants it was possible to correct the deleterious effects of mutation by using a particular single amino acid-substituted analog of the peptide molecule. The observed pattern of complementation led to a model that predicts that the Ag assumes an extended conformation, with a turn, in the binding groove, such that the following residues are in close proximity: DR 86-HA 309, DR 71-HA 312, DR 30-HA 314, and 315.