1. The relationships among myelin encephalitogenic or basic proteins were immunochemically examined. 2. Rabbit antisera to myelin basic proteins isolated from chicken, rabbit, bovine, guinea-pig, and human brain specimens were prepared. By quantitative microcomplement fixation these rabbit antisera were used to measure cross-reactions among the myelin basic proteins of the turtle, chicken, rat, rabbit, cow, pig, sheep, dog, guinea-pig, monkey and human. 3. A spectrum of cross-reactivities was detected even though some cross-reactivity persisted across major phylogenetic barriers. Varying, but sometimes marked, differences existed in reactivities of the small and large basic proteins of the rat. Reciprocity of cross-reactivities among basic proteins was inconstant. 4. This study demonstrates the range of immunochemical cross-reactivities among myelin basic proteins and the sensitivity of quantitative microcomplement fixation in assessing such antigenic or conformational differences.