Tissue sections and single cell suspensions of rat thymus were analyzed by flow and image cytometry to study the effects of different doses of bromodeoxyuridine (BrdU) (10, 50 and 100 mg/kg and different labelling periods (15, 30 and 60 min) on the BrdU-labelling index. The proportions of BrdU-labelled cells, as analyzed by flow cytometry, were similar in the different groups of our study. However, the quality of the results was not exactly the same, since when the BrdU dose decreased, the separation between BrdU-positive S-phase cells and both the G0/G1 and the G2/M BrdU-negative cells on a DNA/BrdU histogram became less clear. On the other hand, the proportion of BrdU-labelled cells in tissue sections was greater in the groups of animals that received doses of 100 mg/kg and were killed 15, 30 or 60 min later than in the sections obtained from the remaining groups. Our results show that both dose and exposure time to BrdU may influence the final results when cell proliferation is assessed, the variations obtained clearly depending on the technique used for the immunological detection of BrdU-positive S-phase cells.