A homogeneous IgA protein was isolated from the serum of a dog with myelomatous disease. Analytical ultracentrifugation showed that approximately 60% of this protein existed as an 11S component and the remainder as higher order polymers. The reactivity of the intact protein with an anti-human “J” chain antiserum implied that it contained a considerable amount of a “J” component. This was confirmed by the isolation of a highly acidic polypeptide by gel-filtration and ion-exchange chromatography from the 11S IgA after reduction. The amino acid composition of this polypeptide corresponded closely to that reported by others for human “J” components.

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