We have characterized the alpha-bungarotoxin receptors (BgtRs) found on the cell surface of undifferentiated pheochromocytoma (PC12) cells. The PC12 cells express a homogeneous population of alpha7-containing receptors that bind alpha-Bgt with high affinity (Kd = 94 pM). The BgtRs mediate most of the response elicited by nicotine, because the BgtR-specific antagonists methyllycaconitine and alpha-Bgt block approximately 90% of the whole-cell current. The binding of nicotinic agonists to cell-surface BgtRs was highly cooperative with four different agonists showing Hill coefficients in the range of 2.3-2.4. A similar agonist binding cooperativity was observed for BgtR homomers formed from chimeric alpha7/5HT3 subunits expressed in tsA 201 cells. Two classes of agonist binding sites, in the ratio of 4:1 for PC12 cell BgtRs and 3:1 for alpha7/5HT3 BgtRs, were revealed by bromoacetylcholine alkylation of the reduced sites on both PC12 BgtRs and alpha7/5HT3 BgtRs. We conclude from this data that PC12 BgtRs and alpha7/5HT3 homomers contain at least three distinguishable agonist binding sites and thus are different from other nicotinic receptors.