Antibodies against apamin, a neurotoxic polypeptide from bee venom were raised in rabbits by immunization with apamin or apamin-BSA conjugates.3H-apamin or125I-apamin were used in radioimmunoassay with anti-apamin for the detection of the apamin antigenic site. The inhibitory activity toward the labelled apamin-anti-apamin binding was maximal with unlabelled apamin and decreased in the range: apamin Cys1, Lys4-disuccinilated apamin Cys1, Lys4-diacetylated apamin Cys1, Lys4-diacetylated apamin with carboxymethylated His18. Dipyrimidyl-Orn13,Orn14-apamin derivative almost had no inhibitory activity on labelled apamin binding emphasizing that Arg13, Arg14are the most essential for the apamin topographic antigenic site. © 1981.