Monocional antibodies which recognize a synthetic peptide derived from influenza virus nucleoprotein in association with a murine class I MHC molecule (Kd) have been isolated. One such antibody has been characterized and shown to react neither with the peptide nor with the Kd molecule, but only with the Kd-peptide complex. Evidence is given that it recognizes the naturally processed peptide located in the peptide binding groove, i.e. the antigenic molety presented to T cells.