Lymphocytes, stimulated with different doses of plant mitogens or allogeneic cells, incorporate varying amounts of [(3)H]thymidine. Theoretically, this may be due to different numbers of responding cells, to earlier proliferative response of these cells, and/or to their more or less rapid transit through the cell cycle. Dog peripheral blood lymphocytes were stimulated in vitro with different doses of phytohemagglutinin (PHA), or with allogeneic lymphocytes. After their synchronization by incubation with hydroxyurea (4 mM), the mean durations of the cell cycle, and of the different cell cycle phases were constant and unrelated to strength or type of stimulation. PHA-stimulated lymphocyte cultures were maintained in the presence of colchicine, to prevent clonal proliferation of responding lymphocytes. DNA uptake in this setting, attributed to first generation responders, was related to the strength of proliferative lymphocyte response in control cultures without colchicine. Furthermore, cell proliferation occurred earlier with greater stimulation. It is concluded that higher [(3)H]thymidine uptake in vitro by stimulated lymphocytes is due to greater numbers of responding cells, which are triggered into proliferative response earlier, and not to a more rapid transit of the responding cells through the cell cycle.