Drug-induced agranulocytosis is a highly individualized and unexpected reaction to specific drugs. It may be due to immunogenic or cytotoxic factors. Most instances are produced by a poorly understood immune response to immunogenic drugs. Others are associated with direct suppression of marrow committed stem cells by the direct action of the offending drug or its toxic metabolic end products. The early appearance of polymorphonuclear neutrophil (PMN) antibodies may offer an early warning to sensitized patients. Antibodies, if present, disappear shortly after the drug is discontinued. Agranulocytosis, due to direct action of the drug, is characterized by morphologic aplasia of marrow and is more likely to occur if the affected host has a concomitant defect in marrow cellular proliferation. Accumulation of toxic metabolic end products such as arene oxides may occur if the host is deficient in a microsomal system required to dispose of this material.