In this study, two different methods of estimating cell proliferation were compared: cell loss and potential growth rate of xenografted head and neck cancer grown in nude mice based on the detection of DNA incorporation of bromodeoxyuridine (BrdUrd) in one method, and [3H]thymidine ([3H]TdR) in the other. The 21-d-old xenografts were labelled in vivo, either with BrdUrd or [3H]TdR and excised at intervals during 65.5 h. In tumors containing BrdUrd, the percent labelling was measured in mid-S and mid-G1 phase windows of cytograms from bivariate DNA flow cytometry (FCM). In [3H]TdR-labelled tumors, the percent labelled mitoses (PLM) was determined by light microscopy evaluation of autoradiographs. With a computer program based on a theoretical model, the percent labelling versus time after injection was used to analyze cell cycle time, cell loss, tumor growth fraction, and potential doubling time. The values calculated from DNA incorporation with BrdUrd agreed well with those obtained from labelling with [3H]TdR, i.e., cell cycle time 2.3 vs. 2.4 d, and growth fraction 67 vs. 70%. The estimated potential doubling time was 3.1 d and cell loss factor 40% by both methods. Flow cytometry analysis of BrdUrd-labelling is considerably faster than the evaluation of [3H]TdR-labelling, and the present results provide further support for the BrdUrd labelling method as a promising alternative to the PLM method in cell cycle studies designed to evaluate the relevance of cell proliferative properties in relation to biological behavior in xenografted head and neck cancer.