This report describes a novel assay for the detection of gastrointestinal anthelmintics using mice infected with Haemonchus contortus and adapted to the 1 animal/test group protocol. Mice infected with both H. contortus and Heligmosomoides polygyrus were fed ivermectin-medicated diets for 6 days. A dietary level of 0.09375 ppm was 98.1% effective against the 0- to 6-day-old abomasal stomach worm of sheep, whereas a level of 0.75 ppm reduced the 3- to 9-day-old H. polygyrus worm burden by 94.0%. H. contortus was approximately 8-fold more sensitive to ivermectin than was H. polygyrus in this model. The sensitivity of this assay rivals that of the gerbil-Trichostrongylus colubriformis model while utilizing a more economical host.