Abstract The proliferative pattern of lymphoid cells in the thoracic duct of the calf was studied by the use of tritiated thymidine and autoradiographic evaluation of lymph samples taken at frequent time intervals. The fact that the thoracic duct contains a considerable number of cells in mitosis makes it possible to define more precisely the time parameters of their generation cycle in one and the same animal. A combined analysis of the mitotic labeling index (MLI) and the mean grain count per labeled mitotic figure (MGC/LMF), both as a function of time after a single i.v. injection of 3H-thymidine, revealed an in vivo G2 period of 30 to 40 min, a DNA synthesis time of 3 1 2 hr and a generation time of 5 1 2 to 6 hr for the majority of cells. The duration of these separate phases of the cell cycle was not changed to any appreciable degree by 48 hr of extracorporeal irradiation of the circulating blood (ECIB) prior to the study of lymphoid cell proliferation.