AbstractThe binding capacity of isolated acetylcholinesterase‐rich nerve‐ending membranes of the cat cerebral cortex for dimethyl‐14C‐d‐tubocurarine (1.5 × 10−6M) was studied with anin vitrotechnique. After treatment of the membranes with organic solvents, most of the radioactivity was found in the extract and bound either to the lipids or proteolipids, but not to the gangliosides.Comparing the binding capacity per milligram of proteolipid protein of theM11.0 nerve‐ending fraction with that of myelin, there was a tenfold increase in specific activity in the former. This layer cromatography of a lipid extract, previously passed through Sephadex G 25 fine, showed that the radioactivity remained at the point of origin together with the proteolipids. The use of a special technique with Sephadex LH20demonstrated that the bound dimethyl‐14C‐d‐tubocurarine was eluted together with a protein peak which had no cholesterol and negligible amounts of phospholipids.Experiments using precipitation with ether of the proteolipid protein and passage through the Sephadex LH20column gave similar results and confirmed that the receptor properties ford‐tubocurarine may be in a special type of proteolipid present in the nerve‐ending membrane, probably at the junctional complex. These results are discussed in relation to previous attempts, reported in the literature, to isolat