The present studies were designed to investigate whether experimental, mixed trichostrongyle infections of stabled calves prior to their first grazing season could confer sufficient immunity to significantly reduce egg excretion after turnout, and thereby prevent loss-producing infections later on. The study comprised four groups, each of seven calves. During spring, two of the groups received two different dose levels of infective larvae twice weekly, and one group received larger larval doses at monthly intervals. One group served as non-experimentally exposed controls. In May all groups grazed separately on similarly contaminated plots. All experimental groups had reduced egg counts, and herbage infectivity of their plots was significantly lower than that of the controls. These findings were reflected in serum pepsinogen levels and in specific serum antibody responses. Additionally, clinical disease was only observed in the control group animals.