A method of performing deoxyuridine suppression tests on bone marrow cells using microtitre plates is described. The geometric mean and 95% reference interval derived from 19 patients with normoblastic erythropoiesis and normal levels of serum B12 and folate were, respectively, 5.1 and 2.0-12.8%. In 70 deoxyuridine suppression tests on 60 patients, the correlation between the results from the conventional tube test and the plate method was good, r = 0.9, P less than 0.001, and replicate tests with both techniques showed equal degrees of precision. The microtitre plate method uses fewer cells and is easier to perform than the tube method: thus each test may be performed in triplicate, and additional investigations carried out on cells obtained from a single marrow aspiration. We conclude that the microtitre plate method of performing a deoxyuridine suppression test is useful for both diagnostic and research purposes.