We have measured plasma levels of an alpha-2-L-fucosyltransferase in 18 patients with acute adult leukemia at various clinical stages along with simultaneous bone marrow aspirations and biopsies. Patients in remission had significantly lower levels of this enzyme than did nonresponding or relapsing patients. Furthermore, plasma levels were correlated with percentage of marrow blast cells. This enzyme may serve as a biological marker for monitoring patients with acute leukemia receiving chemotherapy. It is particularly helpful as a guide to chemotherapy in patients on whom bone marrow aspirates are technically difficult to obtain.