Anthelmintic activities of 22,23-dihydroavermectin B1 (comprised to greater than or equal to 95% 22,23-dihydroavermectin B1a and less than or equal to 5% 22,23-dihydroavermectin B1b) against gastrointestinal nematodes in calves were evaluated in 2 controlled experiments. Infective larvae of Haemonchus contortus, Ostertagia ostertagi, Trichostrongylus axei, T colubriformis, Cooperia oncophora, and C punctate were given to 11-week-old calves and Oesophagostomum radiatum had been given 21 days earlier to allow all larvae to attain adulthood at the same time. Each experiment had 4 groups of 5 calves each. Treatment was given to the calves at 14 weeks of age, and 7 to 8 days after treatment, the calves were necropsied. In experiment 1, group 1 control calves had a geometric mean of 11,054 nematodes; groups 2, 3, and 4 calves (at 14 weeks of age) given 22,23-dihydroavermectin B1 orally at doses of 50, 100, or 200 microgram/kg had reductions of 73.5%, 97.3%, and 99.7%, respectively. In experiment 2, group 5 control calves had a geometric mean of 18,897 nematodes; groups 6, 7, and 8 calves given 22,23-dihydroavermectin B1 subcutaneously at doses of 50, 100, or 200 microgram/kg had reductions of 74.6%, 95.3%, and 98.8%, respectively.