Exposure of the normally cryptic Tn antigen on haemopoietic cells leading to erythrocyte polyagglutination has been reported in a few cases of malignant or premalignant haemopoietic disorders and has been attributed to a selective deficiency of the enzyme 3-beta-D-galactosyl-transferase. A male patient presented with acute myelomonocytic leukaemia with no evidence of Tn expression but, 16 months later, in the terminal stage of the disease, the majority of the erythrocytes were found to be polyagglutinable. Tn expression was confirmed by the use of lectins and by agglutination with a Tn-specific monoclonal antibody, FBT3. Retrospective studies of stored blood and bone marrow smears were performed by immunocytochemistry using FBT3. Tn positive cells were first detected in the marrow 8 months prior to death. They increased progressively in number and, in the terminal illness, over 90% of erythroid precursors in the marrow and erythrocytes in peripheral blood and 40% of granulocyte precursors of the marrow and 10% of granulocytes in the blood were Tn positive. These observations suggest that Tn expression was present in a subclone of cells which became dominant during the course of the disease and that there may be a relationship between Tn expression and leukaemic progression.