Immunoglobulin E (IgE) in serum was measured by an improved quantitative test (an assay with competitive inhibition of the binding of 125I PS-IgE to bromacetylcellulose-anti-ND-IgE). IgE levels were determined in serum samples from 35 postpartum mothers (μ = 205 units/ml, range 19.4 to 810), 33 newborn cord sera (μ = 2.1 units/ml, range <1 to 7.5), 23 6-week-old infants (μ = 5.5 units/ml, range <1 to 31.6) and 17 6-month-old infants (μ = 57.6 units/ml, range 4.1 to 458). In 28 maternal-infant pairs we confirmed the absence of transplacental passage of IgE. Cord serum levels reported here are lower than those from other laboratories; in our study half of the newborns had less than 1.5 units/ml of IgE. No synthesis of IgE was apparent in the serum of one-third (8) of the 6-week-old infants, and only 3 of 23 overlapped the adult range. All 6-month-old infants had measurable IgE levels. The 3 with clinical allergy had 3 of the highest 4 serum IgE levels (47, 69, and 458 units/ml). In the adult group there was a presumptive fit to the Hardy-Weinberg distribution, consistent with simple Mendelian heredity of basal IgE levels.