We have investigated the relationship between cell numbers and the amount of tritiated thymidine ([3H]TdR) taken up by stimulated human peripheral lymphocytes, as a function both of labeling time and of the specific activity of the thymidine. Cells responding either to mitogens or to allogenic cells show simple first order kinetics for the uptake of thymidine. Fitting the data to a Michaelis-Menten type of model, we observe for labeling times of 12 hr and longer, non-competitive inhibition of thymidine uptake by increased specific activity of tritium label, regardless of the mode of stimulation. However, for an individual responder in MLC at any arbitrary but fixed specific activity, dose of [3H]TdR and labeling interval, we still observe a linear relationship between cell mass and incorporated label. In contrast, if specific responding combinations in mixed lymphocyte culture are compared, the inhibition by specific activity at longer time intervals becomes significant and influences the quantitative interpretation of results. Specific activities of less than 10 Ci/mmole and labeling times of 6 hr or less avoid inhibition and ensure a linear relationship between dividing cell number and CPM (counts per minute recorded) of incorporated label.