Relevant to the investigation of genetic factors involved in allergic response is the mode of inheritance of IgE levels, an important factor in allergic disease. Serum total IgE levels in 208 Pennsylvania Amish individuals from 23 nuclear families were studied. Males had a significantly higher mean in IgE level than females. There was also a significant negative relationship between age and IgE levels. Segregation analysis was used to fit a major gene model to the age- and sex-adjusted data. The environmental model was rejected (x2 = 41.12, p less than 0.005). The Mendelian codominant model was not rejected x2 = 8.84, p approximately or equal to 0.03) and gave a better fit to the data than the Mendelian dominant or recessive model.