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The ω-conotoxins are small, disulfide-rich peptides which inhibit voltage-sensitive calcium channels. Biotinylated ω-conotoxins are potentially useful reagents for characterizing distinct subsets of calcium channels. We describe the preparation and characterization of biotinylated derivatives of two specific ω-conotoxins, GVIA and MVIID, which bind different calcium channel subtypes. Eight biotinylated derivatives were tested; all specifically displaced binding of the radiolabeled unbiotinylated ω-conotoxin. In general, the addition of one biotin moiety decreased the apparent affinity for the receptor target site by only ∼ 10-fold. However, derivatization of ω-conotoxin MVIID at the Lys¹⁰ residue caused a much more marked effect, a ca 500-fold decrease in affinity. These results indicate that the vicinity of the Lys¹⁰ residue of ω-conotoxin MVIID may be more critical for binding to the receptor target site than regions around …