IgE antibody levels to Hymenoptera (honey bee, hornet, wasp or yellow jacket) venom or venom sac were measured in the serum of ninety-six patients with a history of immediate hypersensitivity reactions to these insect stings. Normal levels of IgE antibody were found in fourty-four of these patients tested. The severity of the systemic reaction was similar in those patients with normal and elevated IgE antibody levels, and the interval from the systemic reaction to antibody determination was also similar. In twelve patients with a definite reaction to honey bee venom, the IgE antibody was elevated in all and correlated with a positive venom skin test in nine out of the twelve. It is not possible to determine if the other patients with normal IgE antibody and a systemic reaction represent a false negative RAST value, loss of sensitivity to the Hymenoptera venom, or a lack of RAST sensitivity with some venom or venom sac antigens. Without the ready availability of venom skin tests to all Hymenoptera antigens or other in vitro tests, a definite assessment of insect anaphylaxis remains in doubt for the patient and physician.