An expansion in the number of viable cells results in an increase in the overall activity of mitochondrial dehydrogenases in the sample. The internal environment of the proliferating cell is more reduced than that of non-proliferating cells. Specifically, the ratios of NADPH/NADP, FADH/FAD, FMNH/FMN, and NADH/NAD, increase during proliferation. Compounds such as tetrazolium salts and Alamar Blue, which can be reduced by these metabolic intermediates, can be used to monitor cell proliferation. Their reduction is accompanied by a measurable shift in wavelength with a corresponding change in color.