It has been shown that guineapigs can be effectively protected against re-infection by the subcutaneous inoculation of embryonated Ascaris eggs (Soulsby, 1957). The purpose of this communication is to report a successful attempt to immunize guineapigs against a lethal oral challenge dose of eggs by the intravenous inoculation of third stage larvae of A. suum.

Three groups, each containing nine guineapigs, were infected with Ascaris as follows: Group I was given 10,000 eggs by mouth. Group II Was inoculated intravenously with 2,000 third stage Ascaris larvae which had been obtained from the lungs of other guineapigs on the sixth day of infection. Both groups were challenged at the same time with 250,000 eggs by mouth, along with a control group (Group III) which had no previous infection. The guineapigs in Group I were re-infected after 18 days and those in Group II after 12 days.