The use of a variety of probes for neuronal nicotinic acetylcholine receptors1–6 has indicated that there are sites on neurons that bind the acetylcholine receptor antagonist α-bungarotoxin1, but do not regulate cation channels in response to the binding of acetylcholine6,7. Sites with high binding affinity for nicotine and for α-bungarotoxin also show different distributions in brain1. A monoclonal antibody (mAb35) raised against acetylcholine receptors from Electrophorus electric organ has been used to purify receptors from chick brain8. These receptors do not bind α-bungarotoxin but have a high affinity for nicotine9 and antibodies raised against them block the function of acetylcholine receptors in ciliary ganglia10. We have raised a monoclonal antibody (mAb270) against acetylcholine receptors from chicken brain (P.J.W., R. Liu, S. Shimasaki, F. Esch, B. Morley & J.M.L., manuscript in preparation), which has been used to purify a similar receptor from rat brain11. Here we show that this antibody identifies a functional nicotinic acetylcholine receptor in rat-neuron-like PC 12 cells which is probably not encoded by the cDNA clone (λPCA48) previously suggested as a candidate for the acetylcholine receptor gene5. Additionally, we show that mAb270 binds to the same areas of rat brain as nicotine.