Variations in the upper limit of normal total serum IgE have been reported: they can range from 150 to 1,000 UI/ml; but the usually accepted upper limit is between 150 and 300 UI/ml. In this study we measured the total serum IgE levels (RIST-Pharmacia) in 69 randomly selected control subjects; 38 had no previous history of atopic or parasitic disease. The normal values observed were 7-460 UI/ml for the whole of the control group (10th-90th percentiles) and 0.5-540 UI/ml for the 38 selected subjects (5th-95th percentiles). Logarithmic conversion is compulsory in order to obtain a gaussian distribution. This enables 80 p. 100 of the control subjects to be grouped within 4-450 UI/ml or 90 p. 100 of the selected subgroup within the range of 1.5-677 UI/ml. As many allergic patients have a normal serum IgE level and a number of non-allergic diseases may be associated with an increased serum IgE level, the diagnostic importance of this investigation is limited. The cost-effectiveness is very low and raises the problem of using this test in everyday practice.