The efficacy of the avermectin compound 22, 23-dihydroavermectin B1 against murine strongyloidiasis has been examined. A daily dose of 4 micrograms (200 micrograms kg-1) totally suppressed excretion of Strongyloides ratti larvae in the faeces. A similar marked suppression was seen when 10 micrograms (500 micrograms kg-1) of the drug was given on two days during the phase of larval migration or during the intestinal phase. Avermectin did not affect larval numbers in the skin but reduced larval numbers in the lungs by 91%. Administration of avermectin during the phase of larval migration completely prevented the subsequent appearance of adult worms in the gut. A single dose of 50 micrograms (2.5 mg kg-1) eradicated intestinal adult worms. Some variability was noted in dose-response studies, but the drug was very potent and a dose of 50 micrograms (2.5 mg kg-1) always eradicated the worms. Avermectin greatly reduced the numbers of S. stercoralis larvae in the muscles, whether it was given early or late in the infection. Eradication of S. stercoralis larvae from muscle followed a single dose of 100 micrograms (5 mg kg-1). It is concluded that this avermectin could be valuable in the treatment of human strongyloidiasis.