Abstract
1. Subcellular fractions and myelin were isolated from developing and adult rat brain. 2. Measurements of chemical composition and enzyme activities indicate the presence of a second myelin-like fraction mainly in the brain of developing rats. 3. This membrane fraction has a different lipid composition from myelin, but resembles myelin in its content of phosphohydrolase and aminopeptidase activity. 4. It is suggested that the second myelin-like fraction may be a submicrosomal contaminant or it may be derived from oligodendroglial plasma membrane during myelinogenesis.