2-Chlorodeoxyadenosine (cladribine, Leustatin) is being used extensively in the treatment of hematologic malignancies, but relatively little is known regarding its toxicity to the normal marrow. Long-term serial hematologic observations have been made on 29 patients with multiple sclerosis undergoing experimental therapy with monthly courses of cladribine, each of which consisted of 0.087-0.1 mg/kg per day for 7 days. The characteristic hematologic responses of the patients consisted of acute transient monocytopenia, prolonged, profound lymphopenia especially of CD4-positive cells, and modest lowering of the granulocyte count and hemoglobin with development of long-lasting macrocytosis. Two patients developed severe aplastic anemia, requiring transfusion both of red cells and platelets. One of these had previously received extensive therapy with chlorambucil, while the other had received carbamazepine (Tegretol) and was ingesting phenytoin (Dilantin) at the time of cladribine therapy. Both patients recovered after several months of marrow suppression.