A study has been carried out in order to establish the diagnostic value of serum IgE estimation and other in vitro tests in drug allergy. The comparative value of these tests in allergic asthma and rhinitis has also been assessed. Since there were fewer problems in the diagnosis of the latter conditions, serum IgE estimation was of relatively greater value in the diagnosis of drug allergy, although raised IgE was found only in twenty-three of sixty-two (37%) patients. The overall incidence of raised IgE in allergic asthmatics (20%) was lower than in previous reports, and like in drug allergy, the frequency was smaller in patients with a single allergy. In drug allergy, the importance of using proper material in the various tests was illustrated in penicillin allergy. Skin tests and in vitro correlates of immediate-type allergy frequently gave false negative results with the drug itself rather than its conjugated derivatives (penicilloyl-polylysine and penicilloyl-human serum albumin), while in the lymphocyte stimulation (transformation) test this limitation was not as marked. The lymphocyte stimulation test was positive in the majority of patients, both in drug allergy and in allergic asthma and rhinitis, and on the whole did not seem to correlate with the type of reaction (whether immediate or delayed) to in vivo testing.