Ten patients who developed severe generalized reactions following a honey-bee sting were investigated for the presence of specific IgE and IgG antibodies, and for lymphocyte reactivity following in-vitro honey-bee venom (HBV) stimulation. Five of the patients (high responders) showed high HBV-specific IgE and IgG levels, whereas the other five patients (low responders) showed low HBV-specific IgE and IgG levels. Mononuclear cells from the high responder group incorporated significant amounts of 3H-thymidine when activated with pure bee venom, whereas insignificant lymphocyte proliferation was observed in the low-responder group. It is concluded that, amongst HBV-sensitive patients, a group of low responders exists in whom the mechanism of anaphylaxis cannot be explained.