Our work shows that light exposures of cell cultures results in the production of hydgogen peroxide in direct relation to the dose (J/cm²) of light given. However, cell growth is stimulated (with respect to unexposed controls) only at low doses, inhibition occurs as dose is increased reaching maximum inhibition near 50 J/cm²., then at higher doses stimulation of cell growth returns as dose reaches 100 J/cm². We believe hydrogen peroxide production is responsible for many of the positive results attributed to laser therapy.