GRANULOCYTOPENIA is the most common hematological adverse reaction to drugs. If infection develops as a result of severe neutropenia (agranulocytosis), the mortality rate is 20% to 50% even with early and appropriate treatment. Only a few drugs are likely to produce this reaction, but these few do so often enough to demand caution in their use. By appropriate observation of patients receiving these drugs, agranulocytosis can be readily recognized and the number of deaths greatly reduced. The drugs most frequently associated with the development of agranulocytosis are listed in the Table. Themechanismby which drugs produce agranulocytosis is not known. In many instances of agranulocytosis associated with the administration of aminopyrine or sulfonamides, it has been possible to demonstrate antibodies in the patient's serum; these antibodies agglutinate leukbcytes in the presence of the drug. In cases of agranulocytosis associated with other drugs, such as the phenothiazines and the antithyroid