Mice infected with as few as ten eggs of the nematode Trichuris muris are highly resistant to a challenge infection given 5 weeks later. The rate of elimination of challenge larvae is similar in mice immunized by infection with ten or with 300 eggs. Stimulation of immunity to challenge by the low-level infections is dependent upon the persistence of the immunizing worm population and removal of the population at day 14 considerably reduces the level of immunity. Worms developing from a low-level immunizing infection are not eliminated by a ‘self-cure’ response, as is the case with larger infections, and become sexually mature. Adult worms from such an infection are, however, eliminated by the response to a superimposed challenge infection. Repeated infection of mice with small numbers of eggs stimulates an immune response and the worm population is eliminated before patency.