Hematologic examinations made during the past ten months in a base hospital in France on certain gassed patients (mustard gas) offer strong evidence that the deleterious action of the poison on the blood and bone marrow is an important factor in prognosis in many cases. The first change in the circulating blood—apparently due to a stimulus to the bone marrow—is an increase in the erythrocyte and leukocyte count, that naturally varies greatly with the severity of the gassing and the individual's reactive powers. In view of the disproportionate increase of red and white cells, change in Arneth scale, etc., the possibility of this being due to a greater concentration of the blood may safely be disregarded. The leukocyte rise (due to the polymorphonuclear elements) may be as high as 36,000 per cubic millimeter, and in the present series averaged 12,000. Enumeration of the lobes of the nuclei (Arneth count) shows