High levels of IgE antibodies (Ab) to bee venom constituents were mainly found in patients with bee sting hypersensitivity, whereas the sera of bee keepers usually contained high levels of IgG Ab and haemagglutinins, representing blocking antibody activity. In bee keepers there was a positive correlation between the degree of severity of an adverse reaction to bee stings and IgE Ab, a negative one between the severity of reaction and IgG Ab and haemagglutinins, a positive one between the number of stings per season and IgG Ab and haemagglutinins, and negative one between the number of stings and IgE Ab. This suggests a mutual dependence of the production of IgE Ab and blocking Ab in a regularly exposed population. No such correlation was found in bee sting-allergic patients, indicating that other factors influence the severity of reaction in this only sporadically exposed group. The values of IgG Ab and haemagglutinins correlated well with each other, with IgG Ab giving a better correlation with severity of reaction and number of stings than the haemagglutinins.