Neutrophilic agranulocytosis developed in two patients with rheumatoid arthritis within the first five weeks of administration of penicillamine (D-penicillamine). One patient died from sepsis, the other recovered after discontinuation of the drug which was resumed at a lower dose without resultant untoward effects. The characteristic hematological findings were compared with those occurring in the course of agranulocytosis caused by other unrelated drugs. In the light of the pathological and laboratory findings, a cytotoxic effect and not an immunologic mechanism was responsible for the neutrophilic agranulocytosis in the patients described. Previous therapy with adrenal corticosteroids in both patients, leading to adrenal cortical insufficiency, may have played a role in the development of the hematological [ill]oxicity.