Estimates of the potential doubling time (Tpot) of seven human tumor xenografts were made using a cytokinesis-block method. This method is currently being investigated as an alternative to flow cytometric assays using the administration of a thymidine analog in the measurement of Tpot. If perfected, the cytokinesis-block method of measuring Tpot would be advantageous as a predictive assay, in that no label is administered to the tumors in situ. Xenografts were grown in nude mice, and following tumor excision and disaggregation, tumor cells were cultured with the cytokinesis-blocking agent cytochalasin B. The flux of cells through mitosis was marked by the accumulation of multinucleate cells. By counting the total number of nuclei as a function of time, the effective population growth was observed. Tpot values were obtained by fitting suitable exponential least squares curves to the data, with the doubling time indicated by the fitted functions. For the seven tumors studied, a significant spread in growth rates was observed. Tpot values generated by this method ranged from approximately 2 days for a rapidly growing squamous cell carcinoma of the pharynx, FaDu, to approximately 7.5 days for the slower growing glioblastoma multiforme U251-MG. These values are compared with standard 5-iododeoxyuridine Tpot measures and volume doubling times obtained by Perez et al. (Cancer Res., 55: 392-398, 1995) for the same tumor xenografts. Although individual Tpot values varied between these methods, the ranking of the seven tumors in order of Tpot times was the same regardless of method. In addition to estimating Tpot, for each of the tumors, the fraction of clonogenically dead cells that was microscopically apparent, including apoptotic cells and cells expressing micronuclei, was determined as a function of time in culture. Tracking this in vitro cell loss rate provides information on the adjustment of these primary tumor cells to in vitro culture, a factor that needs to be addressed when determining how in vitro measurements of Tpot can be effectively related to in vivo measurements.