Abstract: The invention is directed to a new .mu.-conotoxin named GIIIA. .mu.-Conotoxin PIIIA consists of 22 amino residues and is found in the Eastern Pacific fish-hunting species Conus purpurascens. This conotoxin is a new Na.sup.+ channel blocker and can be used to resolve tetrodotoxin-sensitive sodium channels into three categories: 1) sensitive to .mu.-PIIIA and .mu.-conotoxin GIIIA; 2) sensitive to .mu.-PIIIA but not to .mu.-GIIIA; and 3) sensitive to neither of these two .mu.-conotoxins. In rat brain, binding competition studies between the two .mu.-conotoxins and saxitoxin suggest at least three pharmacologically distinguishable binding sites. Thus, .mu.-conotoxin PIIIA should be a key tool for distinguishing among different sodium channel subtypes.