The present study attempted to induce an acquired immunity against the dog hookworm, Ancylostoma caninum. The experiments were carried out to demonstrate and confirm whether an acquired resistance could be developed by the repeated lowgrade infections with the infective larvae of A. caninum in the abnormal host of mouse.

1. In order to determined the distribution of hookworm larvae in the lungs and liver of mice after inoculation of infective larvae, 54 mice were inoculated with 1,000 larvae and sacrificed in batches daily up to 9 days after infection.

It was found that in all cases the average total number of larvae recovered from the lungs and liver increased at 48 hours after infection, then began to decrease and reached 0 to 1 at the 9th day after inoculation.

2. One hundred fifteen mice were immunized 2 or 4 times at 7 day intervals with 50 infective larvae, followed by challenging infection with 1,000 infective larvae and killed in batches at 48 hours after challenge. The interval between challenge and last immunization were from one to four weeks for each group. Sixty mice as the controls were given only challenging infection without previous immunization infection.

Induced resistance was evaluated by the rate of recovery or the average total number of the larvae recovered from the lungs and liver in the challenged mice, compared with the controls.

It was noted that the rates of recovery in the controls were twice or nearly higher than those in the previously infected mice in all instances and these remained low for the first 7 day intervals between the last immunizing infection and challenge, then gradually increased.

3. From the above observations it is highly suggested that an acquired resistance can be produced by repeated previous infections with the larvae of A. caninum, even in low grade, in the abnormal host of mouse, and evaluated by the rate of recovery of the larvae after challenge.