Patch-clamping and photoaffinity-labeling techniques were used to study the effects of binding of monoclonal antibodies (mAbs) on the function of Torpedo californica nicotinic acetylcholine receptor (nAChR). The rat anti-Torpedo nAChR mAbs examined here are known to inhibit ligand binding to either the high-affinity (mAb 247) or both the high- and low-affinity binding sites (mAb 370 and mAb 387) [Mihovilovic, M. & Richman, D. P. (1984) J. Biol. Chem. 259, 15051-15059; Mihovilovic, M., & Richman, D. P. (1987) J. Biol. Chem. 262, 4978-4986]. Single-channel analysis shows that mAb 247 and the Fab fragment of mAb 247 inhibit the opening of the nAChR ion channel, although they have no effects on the structural transition from the resting to desensitized state as monitored by the extent of decreased labeling by the photoreactive probe 3-(trifluoromethyl)-3-(m- [125I]iodophenyl)diazirine ([125I]-TID). In the presence of mAb 387, the nAChR single-channel amplitude was decreased by 20%, whereas Fab 387 completely inhibited channel opening. [125I-TID]-labeling studies suggest that the mAb 387-nAChR and Fab 387-nAChR complexes are able to undergo the transition between resting and desensitized states. This result confirms that the nAChR can assume a desensitized state without prior channel opening. In addition, mAb 35 and mAb 132, which recognize the main immunogenic region (MIR) of the nAChR, and mAb 370 do not alter either single-channel behavior or labeling patterns. Combining the results from characterization with respect to their epitopes and their effects on agonist (carbamylcholine) and antagonist [alpha-bungarotoxin (alpha-BTX) and curare] binding, these results indicate that mAbs could be used to map functional and structural domains.