The binding characteristics of conotoxin GIIIA purified from the venom of a marine snail, Conus geographus, with regard to electroplax membranes from Electrophorus electricus were studied using a radiolabeled monopropionyl derivative of the toxin (3H-Pr-CGIIIA). 3H-Pr-CGIIIA bound specifically to a single class of saturable binding sites in electroplax membranes with a dissociation constant of 1.1 +/- 0.2 nM and a maximal binding capacity of 11 +/- 2 pmol/mg of protein. The latter value was similar to the number of specific binding sites (10 +/- 2 pmol/mg of protein) for 3H-lysine-tetrodotoxin (3H-Lys-TTX). Monopropionyl CGIIIA and CGIIIA had similar inhibitory effects on the binding of 3H-Lys-TTX (1 nM) to electroplax membranes with IC50 values of 3.5 and 0.9 nM, respectively. The association and dissociation of 3H-Pr-CGIIIA and electroplax membranes were much slower than those of 3H-Lys-TTX and the membranes. Mu-Conotoxins (CGIIIA and CGIIIB) and guanidinium toxins (TTX and saxitoxin) inhibited 3H-Pr-CGIIIA (1 nM) binding to electroplax membranes with IC50 values of 0.6, 1.1, 7.1, and 2.2 nM, respectively. However, several other kinds of neurotoxins and local anesthetics known to interact with Na channels did not affect 3H-Pr-CGIIIA binding. These findings indicate that mu-conotoxins must be classified in the same group of Na channel inhibitors as guanidinium toxins, since they competed with guanidinium toxins for binding sites on the Na channel. The peptide mu-conotoxins should be useful in studies on the functional and structural domains of Na channel proteins.