Intensive research in the author's laboratory over a 10-year period has now culminated in the precise determination of the entire antigenic structure of native hen egg-white lysozyme. The protein carries three antigenic sites. Each site is made up of spatially adjacent surface residues that are not in direct peptide linkage. The residues of each site describe an imaginary line which circumscribes part of the surface topography of the protein and act functionally towards the antibody as if they are in direct peptide bond linkage. The reactivity of each site is fully satisfied by an appropriate surface-simulation synthetic peptide, and the three synthetic sites account for the full immunochemical reactivity of the native protein. Each site is subject to conformational restrictions and exhibits directionality which is a function of side chain orientations. The antigenic sites of myoglobin and lysozyme are compared. It is proposed that antigenic sites of the type found in myoglobin are called "continuous sites", while antigenic sites of the type seen in lysozyme are defined as "discontinuous sites".