An enzyme-linked immunosorbent assay (ELISA) was developed that could measure titres of human IgG antibodies to five different venoms (honeybee, yellow jacket, yellow hornet, white-faced hornet, and wasp), and to honeybee phospholipase A. Changes in specific IgG anti-venom titres were measured in twenty patients that had systemic anaphylactic reactions to insect stings, and ten non-allergic controls. After being stung and prior to treatment all patients had anti-venom IgG titres greater than controls. Treatment with small doses of venom over 1–2 months resulted in prompt rises in anti-venom IgG titres that may represent secondary anemnestic responses primed by prior slings. All patients undergoing venom immunotherapy showed at least 2-fold increases in IgG antibody lo the venoms they were treated with by the time maintenance doses of 100 meg were achieved, with one exception. Significant cross-reactive increases in anti-vespid IgG antibodies to venoms not used for treatment occurred in nine of eighteen treated patients. Overall, ELISA of IgG antibodies lo five venoms allowed clear evaluation of the considerable variation of IgG responses among different patients. We conclude that serial determination of venom-specific IgG titres by ELISA offers an important adjunct to evaluating the results of venom immunotherapy.