The concentrations of venom specific IgE (Phadebas-RAST) and IgG (Phadebas IgG-RAST) were monitored in sera of 22 patients with histories of systemic anaphylactic reactions following insect stings who underwent immunotherapy with venom extracts (bee venom and/or yellow jacket venom). Analysis of the immunological parameters during immunotherapy revealed great individual variation in the degree of response concerning both specific IgE and IgG antibodies. Nevertheless, four typical patterns of immune response could be found. The majority of the patients were able to produce high titers of IgG "blocking" antibodies without an IgE increase. Another group of patients had a marked rise in both specific IgG and IgE following the initial phase of hyposensitization. In contrast, one patient received venom injections without a significant IgG or IgE response. Finally, a small group of patients has a marked increase in specific IgE while synthesis of IgG was not observed. This IgE rise was the cause of systemic reactions in this group after the venom injections. Regular monitoring of venom specific IgE and IgG is useful in evaluating the degree of protection for the patient. Since an increasing IgG/IgE ratio must be obtained during immunotherapy, a knowledge of this relationship serves to adapt the individual treatment schedule.