Several monoclonal antibodies (mcAbs) elicited against the nicotinic acetylcholine receptor (AChR) from Torpedo react also with skeletal muscle AChR. Such mcAbs were used to define antigenic determinants on muscle AChR and to elucidate their effect on muscle AChR functions. Primary chick muscle cultures were used as a model for skeletal muscle. Of the four mcAbs studies only mcAb 5.5, which is directed against the cholinergic site in Torpedo AChR, blocks the binding of α‐bungarotoxin (α‐β) to AChR in chick muscle cultures and inhibits carbamylcholine‐induced sodium transport in these cells. The interaction of mcAb 5.5 with the cholinergic site on muscle AChR demonstrates the conservation of this site. Two mcAbs, 5.5 and 5.34, each of a different antigenic specificity but both directed against conformation‐dependent antigenic determinants, accelerate the degradation of AChR in muscle cultures. From the reactivity of the various mcAbs with Triton‐solubilized and membranous AChR it appears that there are some antigenic differences between the detergent solubilized and membranous forms of the receptor.