Enzymatic studies were performed on erythroblasts obtained from marrows of 2 patients with untreated erythroleukemia. Cytochemically, erythroblasts showed abnormalities of several enzymes involved in carbohydrate metabolism, as well as abnormalities of specific and nonspecific esterases. Electrophoretic analysis of esterases extracted from predominantly erythroid marrows showed strong moderately fluoride-resistant nonspecific esterase activity with α-naphthyl acetate, and weak activity with α-naphthyl butyrate. Isoenzymatic patterns of specific esterase activity in erythroleukemia were indistinguishable from those found in myeloblastic leukemia. The results are consistent with the concept of the DiGuglielmo syndrome in which a preleukemic erythroid disorder may precede the emergence of acute myeloblastic or myelomonocytic leukemia.