Bone marrow and blood smears from patients with acute lymphoblastic and myeloblastic leukaemia prior to therapy were stained for tetrahydrofolate dehydrogenase. Compared with other cytochemical tests this reaction seems to discriminate clearly between cells with myeloid and lymphoid features. Lymphoblasts were intensely positive, the reaction being located asymmetrically at one pole of the cell, while myeloid malignant cells were slightly positive with evenly diffused granules in the cytoplasm. Thus, tetrahydrofolate dehydrogenase reaction should be an effective marker of blast cells indistinguishable by other characteristics, as found in unclassifiable leukaemia.