Total serum IgE measured with the Phadebas PRIST technique was titrated in 117 normal non-allergic subjects, 237 allergic adolescents or adults and 89 non-allergic patients who suffered from asthma, rhinitis, or conjunctivitis. All subjects were of Caucasian origin. In normal subjects, mean total serum IgE was 38 +/- 43 kU/l. This value is exactly the same as that found in a study of Caucasian New Zealanders and very similar to the values found in most U.S. studies. This suggests that the mean total serum IgE concentration is remarkably constant in normal non-allergic Caucasians. The upper limit of the normal range is considered to be 150 kU/l. 38% of allergic patients have total IgE concentrations within the normal range. Some pollen or hymenoptera venom-sensitive patients have a total serum IgE concentration below 20 kU/l. The non-allergic patients had a mean IgE concentration of 94 +/- 93 kU/l, and 25% of them had a total serum IgE above the normal range. Asthmatic patients had higher mean IgE levels than those who were suffering from either rhinitis or conjunctivitis.