Noninvasive imaging should facilitate the analysis of changes in experimental tumors and metastases-expressing photoproteins and result in improved data consistency and experimental animal welfare. We analyzed quantitative aspects of noninvasive imaging of luciferase-labeled tumors by comparing the efficiency of noninvasive light detection with in vitro quantification of luciferase activity. An intensified charge coupled device video camera was used to noninvasively image luciferase-expressing human prostate tumors and metastases in nude mice, after ip inoculation of luciferin. Repeated imaging of anesthetized animals after intervening growth periods allowed monitoring of tumor and metastases development. Comparison of photon events recorded in tumor images with the number of relative light units from luminometric quantification of homogenates from the same tumors, revealed that the efficiency with which light escapes tumors is inversely related to tumor size and that intensified charge coupled device images alone are not sufficient for quantitative evaluation of tumor growth. However, a combined videometric and luminometric approach did allow quantification and was used to show the cytostatic effects of paclitaxel in three different human prostate tumors growing in nude mice.