The results of deoxyuridine (dU) suppression tests performed on 400 marrow samples aspirated over a 3-year period are summarised. High dU-suppressed values were found in all patients with vitamin B12 or folate deficiency, in some patients receiving inhibitors of dihydrofolate reductase, in 4 of 19 epileptics receiving anticonvulsants and in 2 of 21 patients with iron deficiency anaemia. High dU-suppressed values were found even in those vitamin B12- or folate-deficient patients who had serum vitamin B12 and red cell folate levels within the normal range. Conversely, some patients with subnormal serum vitamin B12 levels (including 1 on high doses of penicillin, 1 on anticonvulsant therapy and 2 vegans) and some patients with subnormal red cell folate levels gave normal dU-suppressed values, and were considered not to suffer from the metabolic consequences of vitamin B12 or folate deficiency. All of the patients with megaloblastic erythropoiesis induced by the anti-purine drugs or cyclophosphamide, 4 patients with megaloblastic erythropoiesis associated with primary acquired sideroblastic anaemia, a chronic myeloproliferative disorder or subacute erythraemic myelosis and a proportion of the patients with megaloblastic erythropoiesis associated with anticonvulsant therapy, chronic alcoholism, acute myeloid leukaemia or myelomatosis also gave normal dU-suppressed values. The addition of 1 μg cyanocobalamin per millilitre of marrow culture together with the deoxyuridine caused a significant reduction in the dU-suppressed value in 86% of vitamin B12-deficient patients and the addition of 10 μg pteroylglutamic acid per millilitre of marrow culture caused a significant reduction in 91% of folate-deficient and 57% of vitamin B12-deficient patients.