The results presented indicate that the three main peptide parts of bovine encephalitogenic protein (BEP) cross-react when examined by the macrophage migration inhibition test in guinea pigs and the lymph node cell transformation test in rabbits: 1. Guinea pigs were immunized with peptide 43—88 of BEP and the capillary migration of peritoneal cells from these animals was examined in the presence of the intact protein or various peptide fragments derived from the latter. The complete protein and peptides 1–42 and 92–169 were all capable of inhibiting cell migration, but none of these antigens were as efficient as the immunizing agent (peptide 43–88). 2. Lymph node cells from rabbits sensitized with peptide 1–42, 43–88, or 92—169 were examined using the lymphoid cell transformation test. Cells from animals immunized with any one region always showed a substantial degree of transformation also when stimulated by peptides covering the other two regions of the protein, but the immunizing peptide consistently showed a transforming effect better than that of the other two peptides. 3. Control experiments showed that contamination of the immunizing peptide with other parts of the protein could not explain the observed cross-reactivity between the three main regions of BEP.