Thirteen patients with severe reactions to sweat bee stings were evaluated. Eleven patients had had systemic reactions and two, large local reactions. Skin testing and RAST with venom of other Hymenoptera and whole body extract of sweat bees disclosed negative reactions to both skin tests and RAST of all other insects tested in eight, while three showed sensitivity to honey bee. Only one patient had a positive RAST reaction to whole body extract of sweat bee. Whole body extract of sweat bee causes some irritant reaction when skin testing controls and is not a reliable diagnostic agent even though patients were more reactive than controls. Hypersensitivity reactions to sweat bee stings are distinct and not associated with reactions to other stinging insects. Currently, no reliable antigen is commercially available for diagnosing or treating this condition. Sweat bee venom or other venom antigens might be beneficial.