Determination of specific IgE by RAST is a well-established method in the diagnosis of immediate allergic reactions. In this study, we have compared the RAST with the ImmunoCAP, a novel test system which is based on a new type of solid phase. A total of 123 sera from 111 insect venom-allergic patients (74% female) were investigated. All patients had their diagnosis confirmed on the basis of their history and skin tests with insect venom. The patients' age showed a mean +/- S of 44.2 +/- 14.6 years. The total serum IgE levels ranged from 4-1712 kU/l, with a median of 108 kU/l. The results of specific IgE, as determined by RAST and CAP, showed a significantly higher sensitivity, by almost one class, with the CAP compared with the RAST system. The quotient of specific IgE to total IgE, determined with the CAP system, could not be shown to be an expression of sensitization, compared with the severity of sting reactions (Müller classification (16)). A conversion factor for vespid venom RAST to CAP was calculated from the present data by subtracting the RAST from the CAP values. The mean delta value +/- SD was found to be 0.9 +/- 0.65, with a range from -0.8 to 2.7 and a median of 0.9. The data clearly show the differences between RAST and CAP-RAST classes, indicating that the CAP-system has a higher sensitivity and that patients with a low level sensitization are missed by the RAST method.