Normal mitotic and maturation granulocyte pools in Down's syndrome are evidenced by the finding of normal marrow cellularity and morphology. Normal marrow granulocyte reserves are indicated by a normal response to endotoxin injection. The normal levels of serum muramidase support the concept of a normal granulocyte turnover rate. Therefore, it is considered unlikely that there is a significant abnormality of granulocyte kinetics in Down's syndrome. It is also unlikely that increases in leukocvte enzymes in this condition are due to a relatively immature granulocyte population. Rather, they may reflect some general effect of genetic imbalance.