Radioallergosorbent tests (RAST(s)) have been developed and assessed for the diagnosis of insect hypersensitivity by using a purified allergen from honeybee venom, phospholipase A, and crude yellow jacket venom. Sera from 193 patients positive both by history and skin test to one of these insects were compared with various groups of control sera. Eighty percent of sera from skin test-positive patients were RAST positive; positive RAST were found in 16% of sera tested from skin test-negative patients. A highly positive RAST correlates well with a positive skin test and clinical sensitivity, but serum IgE is not measurable in many patients with mast cell or basophil bound antibody. Since biologically important reactions of antigen with IgE require that the antibody be cell bound, skin testing would be preferred to RAST if one were limited to a single test for the diagnosis of insect allergy.