alpha-Conotoxins, a family of small peptides from the venoms of the Conus marine moluscs, are selective, snake alpha-neurotoxin-competitive antagonists of the nicotinic acetylcholine receptor. A new alpha-conotoxin, SIA, has been purified, sequenced, and synthesized. Cross-linking with bivalent reagents and photoaffinity labeling of the acetylcholine receptor with alpha-conotoxin yield covalent adducts. Surprisingly, cross-linking to other subunits is considerably more efficient than to the alpha subunit. The relative efficiency of photoactivatable cross-linking to different subunits of the receptor is a function of placement of the photoactivatable group on the toxin. Since the structures of alpha-conotoxins can be solved by 2D NMR [see Pardi et al. (1989) Biochemistry 28, 5494-5508; Kobayashi et al. (1989) Biochemistry 28, 4853-4860], this family of toxins should provide a set of new ligands for probing the acetylcholine receptor with considerable precision.