The reproduction of Eimeria vermiformis in different strains of phenotypically normal mice and in mice with various immunological characteristics or defects was compared. In some strains of phenotypically normal mice there were marked differences in oocyst production, both in terms of numbers and in the duration of patency, allowing the strains to be classified as resistant or susceptible to infection with E. vermiformis. These differences were apparent only in primary infections; both types of strain were equally resistant to reinfection. Amongst the strains of mice with immunological deficiencies, reproduction of the parasite was greatest in the athymic (nu/nu) mutants and these mice were completely susceptible to reinfection. Strains of mice with lowered or defective antibody production, or with defective neutrophils and low NK cell activity (bg/bg) were more susceptible than the relevant controls to primary infection but all developed substantial immunity to reinfection. Asplenic (Dh/+) mutants were remarkably resistant to infection.