A protein allergen of the parasitic nematode Ascaris has been purified to homogeneity by immunoaffinity chromatography. It is the most abundant protein species in the parasite's body fluid and has been named ABA-1. The allergen's molecular weight (MW) has been previously estimated at 14,000, but this sizing is currently under re-evaluation. The immunological activity of the protein was intact after purification, as attested by immunoprecipitation and passive cutaneous anaphylaxis. The IgE response to ABA-1 was under major histocompatibility complex (MHC) restriction in the rat, in which only RT1u strains were found to respond following infection with the parasite. The tissue-invasive and intestinal stages of both Ascaris lumbricoides (of humans) and Ascaris suum (of pigs) have an antigen of similar MW to ABA-1 in their secretions or among their somatic antigens. These are antigenically indistinguishable; they were found to have similar amino acid compositions, and their N-terminal amino acid sequences were identical to 41 residues. Finally, the apparent MW, amino acid composition and isoelectric point of ABA-1 all argue for close similarity to the previously described Allergen A of the parasite.