The complement-fixing antibrain antibodies which may be found in patients with multiple sclerosis (MS) belong to several specificities, of which only two are as yet identified. In order to study three of the incompletely characterized specificities, the ontogenetic evolution of the corresponding antigens in mouse brain was followed. MS serum containing antibodies to sulfatide, a relatively myelin-specific glycolipid, served as a control. The studied antigens were virtually absent at birth and accumulated at different rates during the postnatal period. The results give valuable clues for the further study of the antibrain antibodies in MS.