The invention is directed to A-lineage conotoxin peptides, which are conotoxin peptides that have strong homology in the signal sequence and the 3'-untranslated region of the genes coding for these peptides to the sequences in the .alpha.-conotoxin peptides. The A-lineage conotoxin peptides include the .alpha.-conotoxin peptides, the .alpha.-conotoxin-like peptides and the .kappa.-conotoxin peptides, described further below. The .alpha.-conotoxin-peptides generally share a "core" sequence motif. This core sequence is termed the .alpha.3/5 core and is represented as Cys-Cys-Xaa-Xaa-Xaa-Cys-Xaa-Xaa-Xaa-Xaa-Xaa-Cys (SEQ ID NO: 1). The .alpha.-conotoxin-like peptides generally share a core sequence termed the .alpha.4/7 core and is represented as Cys-Cys-Xaa-Xaa-Xaa-Xaa-Cys-Xaa-Xaa-Xaa-Xaa-Xaa-Xaa-Xaa-Cys (SEQ ID NO:2). The .kappa.-conotoxin peptides generally have a core sequence termed the .kappa.7/2/1/3 core and is represented as Cys-Cys-Xaa-Xaa-Xaa-Xaa-Xaa-Xaa-Xaa-Cys-Xaa-Xaa-Cys-Xaa-Cys-Xaa-Xaa-Xaa-Cys (SEQ ID NO:3) .alpha.-conotoxins MII (SEQ ID NO:54) and U002 (SEQ ID NO:10) preferentially bind to neuronal nicotinic acetylcholine receptors, rather than neuromuscular receptors. These latter two conotoxins can be used to diagnose and treat small-cell lung carcinomas, which have cholinergic nicotinic receptors.