The antigenic determinants of bovine myelin encephalitogenic protein were investigated by quantitative complement fixation and hapten inhibition using rabbit anti-monkey protein and anti-bovine protein and purified and characterized fragments of bovine protein. The two regions of bovine encephalitogenic protein containing determinants were sequences 1 to 43 and 90 to 170. One rabbit antiserum recognized a determinant(s) probably in residues 15 to 40 while for another rabbit antiserum the determinant of 1 to 43 resided in 1 to 20. The determinant(s) of residues 90 to 170 involved the region around the tryptophan at position 116. Fragment 44-89, which contains the major encephalitogenic determinant for the rabbit, was virtually devoid of any reactivity with the rabbit anti-encephalitogenic protein. It appears that portions of the protein other than the encephalitogenic site are responsible for stimulation of bone marrow-derived cells and antibody production. In demonstrating selected regions of the protein as sites for antigenic determinants, the present immunochemical studies also suggest that the protein might have a more folded conformational alignment than previously suspected.