We studied 708 adolescents aged 15–17 years in the 9th grade of school in Imatra. Of the eligible population born in 1962 77% were included. All were skin prick tested with 16 extracts from two manufacturers with 12 common allergens, which included pollens, epithelia, mite, house dust and fish. At least one positive, immediate reaction (weal diameter 3 mm or larger) occurred in 49% and at least two positive reactions in 43% of those studied. The boys were observed to be significantly more reactive than the girls. The allergen preparations to which positive reactions were most prevalent were house dust, cat and horse epithelium, and mite extract. Large differences in the prevalence of positive reactions were observed with different preparations of the same allergen. Pollen allergens tended to cause the largest positive weal reactions, and the weal size distribution with some pollens was distinctly bimodal. A scheme for calculating allergen potency in histamine-equivalent-prick (HEP) units is presented. It is noted that the result is greatly dependent on the population group chosen for testing.