IgG and IgE antibody levels have been followed for a period of 2 years in patients receiving immunotherapy with bee and wasp venom. 106 adult patients who had had anaphylactic reactions to wasp stings had initially low IgG antibody levels to wasp venom which rose with therapy (p < 0.001). IgE antibody levels also showed an initial rise but subsequently fell (p < 0.001). The pattern was similar to that previously reported in children who had had anaphylactic reactions to bee stings, but who, after a course of immunotherapy, were able to tolerate stings with impunity. 60 adults who had had anaphylactic reactions to bee stings showed a different pattern, with initially high IgG antibody levels which did not rise further. Since two thirds of this group were beekeepers or members of beekeepers’ families, the high initial IgG antibody levels could have been a response to the frequent stings to which such individuals are prone. The fact that high levels did not protect against anaphylaxis shows, however, that the classical concept of ‘blocking antibody’ is in need of revision.