The development of the BrdUrd assay has greatly facilitated the measurement of proliferation in both human and experimental tumors. The potential doubling time (Tpot) was reported to be the most promising parameter to serve as a predictive assay to select patients for new treatment modalities such as accelerated fractionation and fast neutrons. We compared flow cytometry and immunochemistry applied to three sublines of the Dunning rat prostate tumor with differing differentiation. The anaplastic R3327-AT subline has a labeling index (LI) of 7 +/- 0.5% and 8.8 +/- 3.7 measured with flow cytometry and histology respectively, S-phase duration (Ts) of 8 h, a Tpot of 4.7 days and a cell loss factors (phi) of 15%. Proliferating cells are homogeneously distributed. The moderately well differentiated R3327-HI subline shows a LI of 5% (flow cytometry 5 +/- 0.5%, immunohistochemistry 5 +/- 2%), Ts of 10.7 h, Tpot of 8.9 days and phi of 10%. The well differentiated subline R3327-H has two cell populations (DNA-index 1.6) and a labeling index of 3.7 +/- 0.5% measured with flow cytometry and 2.2 +/- 0.4% with immunochemistry.