The migration inhibition factor (MIF) test detects the in vitro release of lymphokine from lymphocytes in in vitro contact with a drug that had sensitized them in vivo. The specificity and sensitivity of the MIF test in identifying a drug inducing an allergic reaction is presented. The MIF test detected the drugs responsible for 20 out of 21 allergic episodes (95.2%) while the basophil degranulation test detected only eight of them (P less than .001). The sensitivity of a positive MIF test was 95.2% and its specificity was 76.9%. The specificity of a negative MIF test was 94.7%. The positive MIF test assisted the physician in indicating the drugs responsible for an allergic reaction in half of the patients. The drugs for which the MIF test was negative could be considered innocent in 95% of the cases. It is concluded that although the results of the present studies are encouraging, the clinical utility of the MIF test is still limited and improvement of the test specificity is required.