One hundred twenty five allergic patients ranging in age from 0 to 36 years old (mean age 9 years) were tested. Intradermal skin tests were performed on the volar surface of the forearm by injection of a five-fold dilution of histamine dihydrochloride (from 1 x 5(-5) mg/ml to 1 x 5(-2) mg/ml). The wheal diameters increased incrementally with histamine concentration. The whealing capacity of the skin to histamine was found to be significantly lower in patients 0 to 5 years of age. There was no difference in the mean wheal size between patients 6 to 10 and 11 to 15 years of age. The wheal size in patients 16 to 36 years of age were significantly larger than those in the other age groups. The flare diameters also increased incrementally with histamine concentration. The mean diameter of flares was significantly lower in patients 0 to 5 years of age. Concentrations between 1 x 5(-3) mg/ml and 1 x 5(-2) mg/ml of histamine dihydrochloride should be taken for a positive control in intradermal allergen skin tests.