An Australian research group studied infants, the majority of whom (72/79) had either one or two atopic parents. A skin prick test was carried out at the ages of 4, 8, 12, 16 and 20 months. The control group further included 20 adults. The size of the histamine-induced (1 mg/ml) wheal increased significantly from the age of 4 months (1.9 mm) to the age of 20 months (2.7 mm). The average size of the histamine wheal in adults was 3 mm.