Detecting the formation of mineralized nodules in osteogenic cell culture provides a means of assessing mature osteoblast cell function and the status of culture. In the present study, to continuously monitor the formation of mineralized nodules during the entire culture period, different concentrations of two fluorescent dyes ( xylenol orange and calcein blue) were evaluated for their ability to specifically label calcified areas and their toxicity to cells in osteogenic cultures. Results showed that 20 mu M xylenol orange and 30 mu M calcein blue gave rise to distinct fluorescent staining for mineralized nodules, which were correlated exactly with von Kossa and alizarin red S staining at the same locations in cultures. In the assessment of toxicity, both dyes at the aforementioned concentrations did not alter cell viability or change the total DNA content in cultures. To demonstrate the advantage of using these fluorochromes to monitor mineralized nodules formation, consecutive fluorescent images of each staining were recorded at the same location of individual culture over the entire duration. The result indicates that both xylenol orange and calcein blue can provide contrasting fluorescent staining to continuously monitor mineralized nodules formation in living osteogenic cell cultures without deleterious effects.