The structural features of apamin, a natural octadecapeptide from bee venom, enabling binding to its receptor and the expression of toxicity in mice, have been delineated by studying the effects on binding and toxicity of chemical modifications and amino acid substitutions in synthetic analogues. The results obtained indicate that the only hydrophobic residue, leucine at position 10, can be changed to alanine without a significant decrease in the specific activity. The need for a correct conformation has been established and also the importance of Gln-17 and the side chains of Arg-13 and Arg-14 (besides the charge effects). The interaction of apamin with its receptor, a calcium-activated potassium channel, is thus mediated by a precise topology around these three residues. Due to the ability to detect very low specific activities for some of the analogues, it has been shown that, individually, none of these interactions constitute an essential criteria for binding per se, but that their presence is necessary for the high specific activity of the toxin.