After the failure of interferon monotherapies for head and neck cancer, an increasing number of combination experiments with chemotherapy and interferons have been carried out to improve the antiproliferative effect. In vitro drug testing requires sensitive assay to detect synergistic effects of tested combinations in order not to get false-negative results. The widely used MTT assay (3,4,5-dimethylthiazol-2,5-diphenyltetrazolium bromide) and the BrdU assay 5-bromo-2-deoxyuridine were compared. It could be shown that the BrdU assay is over 10 times more sensitive and that it reflects the cell proliferation status as shown by flow-cytometric analysis using double staining with BrdU. Labeling with BrdU also allows the estimation of the S phase time. Because of the direct effect of interferons on the cell cycle, the BrdU assay could be appropriate for proliferation studies using interferons.