We describe a simple approach for finding identical amino acid clusters on the outer surface of α -helical coiled-coil proteins by examining the sequence of amino acids that compose the protein. Finding such similarities is an important immunological problem, since these may correspond to cross-reactive epitopes, i.e., sites at which antibodies produced against one protein also bind to another conformationally similar protein. Because of the regularities inherent in a coiled-coil structure the position of each amino acid on the structure is predicted. Based on this prediction, our algorithm finds similarities on the outer surface of the proteins. The matches found by our algorithm serve as an important screening process, intended to indicate which experiments to conduct to determine sites that correspond to cross-reactive epitopes. The location of several cross-reactive epitopes between M proteins and myosins had been verified experimentally. Although our approach makes many simplifying assumptions, these epitopes always correspond to clusters of identical amino acids, which our algorithm predicted to be contiguous on the outer surface. Our algorithm runs in O(n+m+r) time and O(n+m) space, where n and m are the lengths of the protein sequences, and r is the number of matching amino acids that appear in the same structural position of the α -helix in both sequences.