Rats of a Sprague-Dawley strain were infected with embryonated Ascaris Suum eggs. Infected rats examined ‘postmortem’7 days after infection showed characteristic pathological changes in the liver and lung. Most of the sera from infected animals had reaginic antibodies in titers ranging from 1:160 after a primary infection to 1:4,000 after a second infection. The reaginic antibodies were detected by homologous PCA reactions using Ascaris crude extract or a purified Ascaris allergen (Asc-1) as challenge. The results indicated therefore that Asc-1 was present in all stages of the parasite life cycle and was directly involved in stimulating reagin production during the migratory phase of the infection.