The encephalitogenic activity of different parts of bovine encephalitogenic protein (BEP) has been tested in guinea-pigs, mainly at a dose equimolar to 32 mug of intact protein. A high activity was found with complete protein and with region 89-169; both peptide 43-115, tyr (modified at the tyrosine residue 67) and HNB-89-169 (modified at the tryptophan residue 115) showed a moderate but clear-cut activity, whereas that of peptide 1-42 was very faint. No qualitative differences in the disease induced by the different peptides could be found; in only one animal out of 139 were clinical signs observed without evidence of histological disease. Slight histological changes, similar to those seen at EAE, were found in four out of 19 animals injected with Freund's complete adjuvant only.