The RAST, originally developed for the assay of reaginic IgE antibodies, was applied for the detection of blocking antibodies. Sera taken ante- or post-hyposensitization were incubated with solid phase coupled allergen which neutralized reaginic IgE antibodies in the serum. The remaining activity of the reaginic IgE antibodies was measured by the RAST. When blocking antibodies were present in the serum they competed with the reaginic IgE antibodies and the amount of reaginic antibodies that was neutralized by the solid phase coupled allergen was decreased. The blocking antibodies were found in the IgG fraction of the serum taken post-hyposensitization. Blocking antibodies to various allergens were detected in 25 out of 26 tests on sera taken post-hyposensitization. The results were compared with those of Prusnitz-Küstner neutralization tests and/or patient self-neutralization tests. There was no correlation between semi-quantitative results of these in vivo tests and those of the RAST neutralization tests. However, cases with subjectively a strong improvement had a significantly higher mean value for RAST neutralization index than those with subjectively a weak improvement. The method is suggested as a supplement to clinical tests for evaluation of hyposensitization treatments.