Ivermectin B1b (dihydroavermectin B1b) is the minor component (<20%) 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 avermectin/milbemycins are also potent insecticides. In vitro, the B1b (25-iso-propyl) analogue is slightly more potent than the 25-sec-butyl (B1a) analogue as an inhibitor of nematode larval development and paralysis, and also a more sensitive probe for ivermectin resistance.