A new method is described for localizing antigenic determinants in globular proteins by means of inhibition of enzyme-linked immunosorbent assay (ELISA) with tryptic peptides. The inhibitory activity of 10 tryptic peptides of the coat protein of 2 strains of tobacco mosaic virus (TMV) was demonstrated by inhibition of ELISA and verified by inhibition of complement fixation tests. These results bring to 7 the number of antigenic determinants of TMV protein that have been defined.