omega-Conotoxin GVIA (omega-CgTx-VIA) is a 27 amino acid peptide from the venom of the fish-hunting snail, Conus geographus, that blocks voltage-activated Ca channels. The characterization of a biologically active, homogeneous 125I-labeled monoiodinated Tyr22 derivative of omega-conotoxin GVIA and its use in binding and cross-linking studies are described. The 125I-labeled toxin is specifically cross-linked to a receptor protein with an apparent Mr of 135,000. The stoichiometry between omega-conotoxin and nitrendipine binding sites in different chick tissues was determined. Skeletal muscle has a high concentration of [3H]nitrendipine binding sites (greater than 1000 fmol/mg) but no detectable omega-conotoxin sites (less than 7 fmol/mg). Brain microsomes have both binding sites, but omega-conotoxin targets are in excess. These results, combined with recent electrophysiological studies (E. W. McCleskey, A. P. Fox, D. Feldman, L. J. Cruz, B. M. Olivera, R. W. Tsien, and D. Yoshikami, unpublished results), define four types of Ca channels in chick tissues, N, T, Ln (omega sensitive), and Lm (omega insensitive), and are consistent with the hypothesis that the alpha-subunits of certain neuronal Ca2+ channels (Ln, N) are the molecular targets of omega-conotoxin GVIA.