RAST to venoms were done on 108 sera. Positive RAST to one or more venoms of Hymenoptera found in 51% (41/80) patients with a generalized reaction to the sting of Hymenoptera and in 7% (2/28) of normal controls were critically evaluated. RAST determinations for each sera were done in duplicate and results averaged. The laboratories were not told which sera belonged to patients with allergy to Hymenoptera and which belonged to controls. In ten patients, one half of the original sera were saved and RAST to venoms were repeated approximately 1 years later. Of our positive RASTs, 49% (20/41) had positives to more than one type of venom. The most frequent positive RAST was to yellow jacket followed by hornet, wasp, and honey bee. A high degree of cross-reactivity occurred between venoms of hornet, wasp, and yellow jacket. The frequency of positive RAST was similar in pre-treatment and post-treatment sera from patients on immunotherapy with whole body extract. Specific IgE (RAST) results to venoms did not correlate with the degree of clinical severity of the sting. However, IgG anti-venom antibodies were not considered. Elevated RAST titres were associated with increased total IgE levels in sera of non-atopic patients. A positive RAST to venoms is frequently found in sera of patients stung within 5 years. RAST scores of 2 to 4 were remarkably reproducible, while scores of 1 were not consistently reproducible. RAST results should be interpreted in conjunction with the clinical history.