The distribution of proteolipids (PL) was studied in various parts of the nervous system of certain mammals and subcellular structures of rat brain. Soluble PL of various degrees of purification from lipids have been obtained from gray and white matter of cerebral hemispheres of cattle as well as from mitochondrial, synaptosomal and two myelin fractions of rat brain. Certain properties, protein and lipid composition and molecular weight of the PL isolated have been studied. Together with some general properties certain differences have been observed in the protein and especially in the phospholipid composition of PL isolated from the grey and white matters and subcellular particles. The PL of white matter and myelin are similar to each other in their protein and phospholipid composition but are different from those of grey matter and mitochondria, the latter in turn being also similar in their protein and phospholipid composition. The data obtained make the existence of at least two types of PL probable: one in myelin and another in mitochondria. The first is characterised by the presence of a protein corresponding to DM-20 of molecular weight of 29000, besides the main PL protein of 35-36000 molecular weight, as well as by the predominance of phosphatidylserine which forms 61% of phospholipids bound to PL. The second is characterized by the existence of one main protein corresponding to PL but of a somewhat higher molecular weight (39--40000). The phospholipid composition of the PL is rich in cardiolipin which form 38--46% of bound phospholipids.