Phenol Red sodium salt is commonly employed as a cell tissue culture pH marker. Under normal conditions, phenol red added to the growth medium will have a pink-red color. Dying cells or overgrowth of contaminants will cause a change in pH resulting in a change in indicator color. Phenol red bears a structural resemblance to some nonsteroidal estrogens and has significant estrogenic activity at the concentrations (15-45 μM) at which it is found in tissue culture media. Phenol red binds to the estrogen receptor of MCF-7 human breast cancer cells and stimulates the proliferation of estrogen receptor-positive MCF-7 breast cancer cells in a dose-dependent manner but has no effect on the growth of estrogen receptor-negative MDA-MB-231 breast cancer cells. Phenol red is a simple, rapid and inexpensive method for the measurement of hydrogen peroxide produced by cells in culture. The assay is based on the horseradish peroxidase-mediated oxidation of phenol red by hydrogen peroxide which results in the formation of a compound demonstrating increased absorbance at 610 nm.