Ivermectin B1a (dihydroavermectin B1a) is the major component (>80%) of the commercial anthelmintic, ivermectin. Members of the avermectin/milbemycin anthelmintic class exert their anthelmintic effects by binding to glutamate-gated chloride channels expressed on nematode neurones and pharyngeal muscle cells. The avermectins and milbemycins are also potent insecticides. The individual 25-sec-butyl (B1a) and 25-iso-propyl (B1b) components of ivermectin have received little separate study.