A simple method for the estimation of low serum IgE levels is presented. Radioimmunoassay often gives falsely high values because of the presence of non-specific factors in the serum when an excessive amount of serum is used in order to measure low serum IgE levels. Therefore the serum sample has to be diluted to minimize the influence of the non-specific factors and the lowest IgE level measurable in the serum is about 5 u/ml. In this study, pretreatment of the serum with cellulose powder was found to remove the non-specific factors that yield falsely high values. These factors were adsorbed almost completely onto cellulose powder when the serum IgE level was less than 100 u/ml. Thus, low serum IgE levels could be determined by radioimmunoassay using a sufficient amount of serum pretreated by cellulose powder. The lowest serum IgE level measurable by this method was as low as 0.5 u/ml and the low IgE level in neonatal serum could be determined with sufficient accuracy. By this method the geometric level of IgE in cord serum was 2.45 u/ml with a +/- 2 standard deviations range of 1.04 to 5.80 u/ml.