Calf epg data from day of slaughter, and estimated numbers of adult worms recovered at necropsy from calves before (31 d.p.i.), and 10 days after (45 d.p.i.) s.c. injection with ivermectin (Ivomec®, Merial). Calves were previously each infected with C. oncophora and O. ostertagi, representing isolates with different deworming history. Calves in group A received equal mixtures of susceptible laboratory maintained C. oncophora and O. ostertagi from TiHo, whereas calves in groups B and C received cattle nematodes from two different farms in Uppland, Sweden, showing phenotypic clinical IVM resistance in previous field trials.