Recently a method was introduced by Begg et al. (Cytometry 6:620-626, 1985) for the calculation of the DNA synthesis time, TS using flow cytometry to monitor the progression of cells that have been pulse-labeled by bromodeoxyuridine (BrdUrd). Ideally, the method uses only a single two-parameter histogram of the progressed cells to obtain TS. The essence of the method is to be able to obtain an initial point from general principles, draw a line through it and the measured point, and extrapolate to a level of progression that would be attained when the time is equal to TS. In this comment, explicit values are derived for the initial point and the progression of cells based on kinetic arguments, and a new method is presented for estimating TS. This new method is compared to earlier methods, and the effects of a finite labeling time and variation in the transit times are considered. Finally, an equation is developed for estimating TS following multiple measurements.