Tamarins, miniature monkeys of the marmoset family of South America, were passively sensitized by intradermal injection of reaginic sera from patients with atopic allergy to penicillin, dog epidermis, and castor beans. The passively sensitized sites exhibited extravasation of intravenously injected Evans blue dye when the animals were challenged with the appropriate allergens. Sensitivity of the allergic serum transfer test in marmosets appears to be less than in the higher monkeys such as the capuchin and the Old World macaques. It would appear that marmosets could be selectively bred for light-skins and high sensitivity for passive sensitization with human atopic reaginic sera. They are easily maintained and almost always bear twins.