Avermectins are macrocyclic lactones produced as fermentation products of Streptomycesavermitilis. Structurally similar to the macrolide antibiotics, they are effective at extremelylow concentration against a wide variety of nematodes and arthropods. The avermectins appear to induce neuromuscular paralysis by acting on a receptor of the parasites-peripheral neurotransmitter, gamma-aminobutyric acid (GABA). In mammals, GABA is confined to the central nervous system, and because the avermectins do not cross the blood–brain barrier in significant concentration, they do not appear to produce significant untoward effects in the mammalian host. Ivermectin, a derivative of avermectin B1, is currently the drug of choice for the treatment of onchocerciasis and is undergoing evalua-tion for the treatment of other human filarial infections. Its usefulness in other parasitic infections of humans remains to be established.