Skin tests represent a major tool in the diagnosis of reaginic allergy; however, their interpretation does not appear to be without difficulty in children under the age of 3 yr. Seventy-eight infants from birth to 24 mo were prick tested and compared with 30 nonallergic adult subjects. Skin tests were performed without bleeding by use of two strengths of histamine hydrochloride (1 and 10 mg/ml), a mast cell degranulating agent (codeine phosphate, 50 mg/ml), and allergenic extracts. Negative control solution elicited a small wheal (less than 1.5 mm) in two infants who were excluded from further results. A clear and significant (p less than 0.001) hyporeactivity to both histamine and codeine phosphate was observed in infancy, especially before the age of 6 mo. Six infants were allergic and presented positive prick tests to either food or inhalant allergens. These tests were confirmed by serum specific IgE and a suggestive clinical history. The size of the allergen-induced prick test wheal ranged from 2 to 5 mm in diameter, suggesting that prick test wheals may be smaller in infants. This study confirms that prick tests can be performed and interpreted without difficulty in infants, keeping in mind the small wheal size induced by both positive control solutions and allergen-induced prick tests.