After infection with the nematode Trichuris muris 70–75% of mice of the Schofield strain developed an immunity to the parasite and eliminated the worms between the 16th and 19th days after infection. In these mice the acquired immunity persisted for at least 3 months and prevented the establishment of subsequent infections.In 25–30% of the mice immunity was not produced and infections developed into mature worms. These non-resistant mice remained susceptible to further infection.The development and action of the immune response were suppressed completely by the administration of cortisone acetate.