Ivermectin belongs to the macrocyclic lactone class of avermectins and consists of a mixture of two homologous compounds, ivermectin B1a (not less than 80%) and ivermectin B1b (not more than 20%). Ivermectin B1a, the major component of Ivermectin, is inactive at a concentration of 0.3 μg/ml, whereas in the case of the minor component, ivermectin B1b, the same concentration produces 100% mortality of snails.