Penicillin has become a leading cause of allergic reactions, which range in severity from mild pruritus to sudden death. Testing for sensitivity to penicillin is necessary, but hazardous, since occasional fatalities from the skin test itself have been recorded. An in vitro test has therefore been developed. This indirect basophil degranulation test requires one drop each of the patient's serum, of the penicillin solution, and of rabbit leukocytes (buffy coat). A positive reaction is a progressive degranulation observed in the basophil cells. In severe reactions the cell granules are extruded slowly or even explosively. The method has been tested and contratested in many patients. It can be extended not only to the newer synthetic penicillins but also to other drugs such as the sulfonamides and local anesthetics.