A technical modification was introduced by the manufacturer (Pharmacia) of a commercially available RAST, with the use of an anti-IgE antibody specific for the domain ε2 of the IgE molecule. Results of the previous and of the new RAST were compared in 100 patients suffering from bronchial asthma or chronic rhinitis. When the same serum was assayed by both techniques, the new RAST technique disclosed a higher positivity in 26% of the comparisons. The new RAST improved the overall correlation between RAST and skin-tests by 6%: individually by 12% for house dust, 8% for Dermatophagoides pteronyssinus and 15% for animal danders. However, the level of correlation was decreased by 8% for the moulds. In patients with a total IgE concentration larger than 100 PRIST units, the new RAST allowed the demonstration of specific IgE in a number of cases twice as large as the previous technique. Thus, the modification of the RAST provides a more sensitive test, the results of which better correlate with those of other diagnostic methods.