Venom-specific IgE and IgG antibodies were measured in the sera of bee-venom-sensitive patients during a 3-year hyposensitization period. The level of specific IgG antibodies initially increased, and 2 months after the start of therapy, this increase was on average five-fold. A concomitant but non-significant increase in specific IgE antibodies was also observed initially. Later during the treatment period specific IgE antibodies showed a continuous decline and after 3 years the level was one third of the pre-treatment level. Venom-specific IgG antibodies, however, remained above the pre-treatment level. The disappearance of the severe allergic reactions was related to the level of both IgE and IgG antibodies as demonstrated in seventy-six bee sting challenges. In patients with specific IgG antibody levels above 400 u/ml, no severe reactions were observed even if the patient had levels of specific IgE antibodies of RAST class 3 or 4. These data suggest that the relationship between venom-specific IgE and IgG antibodies permits an evaluation of the state of immunity to insect stings.