Muscarinic acetylcholine receptors in a plasma-membrane fraction derived from mouse neuroblastoma clone NIE-115 bind [3-3H]quinuclidinyl benzilate according to the Law of Mass Action (Kdissociation 40 pM, h0.96). 2. Antagonist and agonist binding to the receptor was studied by displacement of [3-3H]quinuclidinyl benzilate with non-radioactive ligands. The data show good agreement with similar data obtained on rat brain and ideal smooth muscle [Birdsall & Hulme (1976) J. Neurochem. 27, 7-16] indicating that the receptor is very similar in these three tissues.