IgE and IgG antibodies to bee venom were measured in sera of patients receiving bee venom immunotherapy. All patients selected for therapy had suffered severe reactions to bee stings. The results showed that within 2-3 months from the commencement of immunotherapy there was a marked rise in IgG antibodies and a slight but not significant rise in IgE antibodies. After this period, the IgE antibody level began to fall and was about one third of the pre-treatment level by the second to third year. The IgG antibody level began to decline from its increased level after 9-10 months but remained above the pre-treatment level even after 2 years. All of the patients who had subsequently been accidentally stung after reaching the maintenance dosage of bee venom allergen showed no severe reactions. A small group of non-treated patients of the same category as those receiving therapy did not show a rise but a decline in the level of IgG antibodies. However, like the treated group, they showed a significant decrease in IgE antibodies. Thus one of the main benefits of bee venom immunotherapy is the build-up of a high concentrations of IgG antibodies, and this may be the critical factor in the protection against bee venom allergy.