An allergenically active fraction was isolated from ragweed pollen which was 50 times as active, on a weight basis, as the dialyzed residue of the whole water soluble extract of ragweed pollen. This material consisted of a straw colored pigment and peptide(s) composed of eight amino acids: lysine, arginine, glycine, glutamic acid, hydroxyproline, valine, alanine and norleucine. It did not contain any carbohydrate. The allergenic activity was destroyed after hydrolysis with 6 N HCl at 100°C for

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... 5 min, but was not diminished on standing in 6 N HCl at 25°C for 8 hr or if heated in boiling water at pH 7.0 for 14 hr. The active material gave positive ring tests with both rabbit and goat antiragweed pollen sera. The available evidence suggests that peptide(s) may account for all the allergens in ragweed pollen.