Nicotinic acetylcholine receptor (nAChR) purified from rat brains by cholinergic ligand affinity chromatography was characterized. Monoclonal antibody 299, which binds an acetylcholine (ACh) binding subunit termed alpha 4, depleted more than 85% of [3H]ACh binding activity of the purified preparation. A number of cholinergic agonists strongly inhibited [3H]ACh binding to the purified nAChR, whereas potential antagonists were less effective than the agonists. These results show that most of the purified nAChR contains alpha 4 subunit and the pharmacological properties are preserved upon purification.