The in vitro incorporation of various radioactive precursors into the DNA and RNA of nucleated bone marrow cells was studied using marrow aspirates from patients with (1) normal blood counts and normoblastic marrows and (2) megaloblastic haemopoiesis due to vitamin B12 or folate deficiency. Although earlier studies employing 0.01 μM 3H-thymidine had indicated that the incorporation of 3H-thymidine was subnormal in some patients with vitamin B12 or folate deficiency, this finding was not seen with higher concentrations of thymidine. In fact, the rates of incorporation of 3H-thymidine (0.22 and 12.5 μM) as well as of 14C-glycine, 14C-formate and 14C-adenine into DNA were found to be abnormally high in some patients with megaloblastic haemopoiesis. By contrast, the incorporation of 3H-deoxyuridine into DNA per 106 DNA-synthesising cells was found to be similar in the normoblastic and megaloblastic groups. Nevertheless, the ratios of the geometric mean for the incorporation of 3H-deoxyuridine to the geometric means for the incorporation of the other radioactive precursors (all expressed as cpm per 106 DNA-synthesising cells) were 2.34–3.92 times higher in the normoblastic than in the megaloblastic marrows, giving direct support to the idea that there is some impairment of the methylation of deoxyuridylate in vitamin B12- or folate-deficient marrows. No abnormality was detected in the rates of incorporation of 14C-glycine, 14C-formate, 14C-adenine and 3H-deoxyuridine into RNA when the data were expressed as cpm per RNA absorbance unit.