SummaryTen patients with malignant lymphocytic lymphoma (diffuse or nodular) have been studied in the leukaemic phase of their disease. The leukaemic cells of the blood and in some cases the bone marrow, a lymph node or a pleural effusion were labelled with tritiated thymidine in vitro and/or in vivo. This made it possible to show that in the peripheral blood, the fraction of cells in DNA synthesis (S) is greater than in the“true”myeloblasts and lymphoblastic leukaemias. The“S”fraction was also found to be variable from patient to patient and in any one patient from one site to another (blood, marrow, lymph node, pleural effusion).The labelling of these cells made it possible to study the mode of action of Vincristine on the cell cycle, twenty-four hours after an injection of 2 mg/m2. The % of lymphoma cells labelled and/or blocked in mitosis when compared to that of the erythroblasts in the same marrow sample, did not suggest a preferential effect on the malignant cells. Moreover, the % of cells labelled with tritiated thymidine in vitro was not compatible with a process of recruitment of lymphomatous cells in“S”by Vincristine.The effect of cytosine arabinoside was studied in one case. It was found to destroy a great number of lymphoma cells without decreasing the proliferative activity which took place at the same time in the surviving lymphoma cells.