A normal rate of incorporation of tritiated thymidine (3H-TdR) into the DNA of bone marrow cells was found in 9 iron-deficient patients and 3 patients with chronic liver disease. By contrast, the uptake was significantly depressed in 5 out of 7 patients with vitamin B12 or folate deficiency, in 2 of 8 patients with megaloblastic changes induced by antipurines and in 2 of 8 patients with a macrocytosis caused by alcoholism. The low rate of uptake of 3H-TdR in the latter 4 cases could not be attributed to an interference with vitamin B12 or folate metabolism as their marrow cells showed a normal suppression of 3H-TdR uptake after pre-incubation with deoxyuridine. 3 patients with a combination of iron and vitamin B12 deficiency showed a normal incorporation of 3H-TdR/103 DNA-synthesizing cells, but the mean rate of incorporation in this group was depressed by 50% following iron therapy. These 3 patients also showed a subnormal suppression of 3H-TdR incorporation after incubation with deoxyuridine, and this abnormality was made worse after therapy with iron. These results indicate that the disturbance in DNA synthesis which occurs in vitamin B12 deficiency is ameliorated by the co-existence of iron deficiency.