Ivermectin has been tested against the microfilariae of Onchocerca lienalis, Brugia pahangi and Dirofilaria immitis in vitro and in vivo. All in vitro tests were performed on larvae incubated for 48 hours at 37 degrees C in Hepes buffered medium 199 containing 20% serum, benzylpenicillin and streptomycin. In vivo tests were performed on larvae in female BALB/C mice dosed with ivermectin, 5 mg/kg, orally. The microfilariae of B. pahangi in vitro were insensitive to ivermectin at concentrations to 30 ng/ml. In vivo, an 87% reduction in the level of microfilaraemia was obtained by 24 hours after drugging but no reduction was observed in the numbers of peritoneal microfilariae. O. lienalis microfilariae in vitro were killed by ivermectin at 3 ng/ml and the larvae of this species within the subcutaneous and cutaneous tissues of the mouse were also eliminated by ivermectin at 5 mg/kg. D. immitis larvae within the bloodstream of the mouse were also sensitive to ivermectin at the dosage employed but were unaffected by ivermectin in vitro at concentrations up to 30 ng/ml
