To obtain information about the normal range of IgE concentration in serum a random population of male and female patients was divided into 14 age groups. Based on the mean values (x +/- 1s) an age dependency is clearly recognizable with an increase of the IgE concentration at one and two years of age, a constant IgE concentration during the middle age period, and a decrease of the IgE level in the 40th year of age which becomes especially obvious after the 60th and 80th year of age. From the distribution pattern of IgE levels within the different age groups it is evident that only the curves of newborns and persons over 80 years resemble a normal Gaussian distribution. The other age groups show 2 or 3 peaks of IgE. However, these peaks represent only 50% of the total IgE concentration. A similar distribution pattern was obtained by IgE determinations in 100 "healthy" blood donors which showed no clinical evidence of allergy. The age dependent normal ranges given in this paper make it possible to evaluate increased IgE levels accordingly.