Deoxyribonucleoside triphosphate pools have been measured in nucleated bone marrow cells from normal subjects and from patients with leukaemia. The deoxyribonucleoside triphosphate (dNTP) concentrations were increased approximately 3-fold in patients with leukaemia and other myeloproliferative diseases. The dNTP concentrations in bone marrow from patients with drug-resistant acute leukaemia were also measured, and in general they were lower in resistant than sensitive marrow cells, although the deoxycytidine triphosphate level was relatively increased in the resistant cells. dNTP concentrations have also been measured in cultured human leukaemic cells and phytohaemagglutinin-stimulated normal lymphocytes. As in other cells, the dNTP concentrations have been noted to fluctuate during the cell cycle and in relationship to the cellular proliferative rate. Measurement of dNTP concentrations in human bone marrow and leukaemic cells is practical but there are major problems relating to the preparation of bone marrow for dNTP measurement. Further studies of dNTP pool changes in animals following drug treatment in vivo are necessary in order to improve methods of preparing cell populations for these biochemical measurements.