The thymidine kinase activity per 10(6) DNA-synthesising marrow cells and the rate of incorporation of tritiated thymidine into the DNA of 10(3) DNA-synthesising marrow cells were estimated in 9 haematologically normal patients and 49 patients suffering from a variety of haematological disorders. Slight increases in thymidine kinase activity were found in 6 of the 31 patients with haematological diseases associated with normoblastic erythropoiesis and greater increases were found in 3 of the 18 patients with megaloblastic haemopoiesis due to vitamin B12 or folate deficiency. In the latter group, there was a statistically significant inverse correlation between haemoglobin levels and thymidine kinase activity. No correlation was found between thymidine kinase activity and the rate of incorporation of tritiated thymidine in either the normoblastic or megaloblastic group, suggesting that the level of thymidine kinase activity does not limit the rate of incorporation of exogenously supplied thymidine into the DNA of human bone marrow cells.