Developmental appearance and accumulation pattern of myelin basic protein (MBP) isoforms were analyzed by quantitative immunoblotting in central nervous system of three mammalian (guinea pig, rabbit and bovine) and one avian (chicken) species. In these four species, myelination onset occurred in the spinal cord well before birth. In addition to the 18.5-kD MBP observed in all species studied, a 21.5- and a 20.2-kD MBPs were detected in the three mammalian species but not in chicken. In calf and chicken, a 17.3-kD MBP was also observed. The 18.5-kD and 17.3-kD MBPs were the major MBP isoforms of chicken central nervous system where a faint MBP-related 14.5-kD protein could also be seen. The major difference between mammalian and avian MBP profiles was indeed the presence only in mammals of the 21.5-kD and the 20.2-kD MBPs. The development patterns of these two isoforms as well as their rate of accumulation as compared to the 18.5-kD MBP suggest that their role in mammalian myelination may be of greater importance at early rather than late stages of this process. Differences in quantity as well as type of MBP isoforms present may indicate that in diverging animal species, the process of myelination may follow a different pathway and possibly involve different MBP isoforms at different stages.