The simplicity and sensitivity of this method of study make it especially valuable for exploratory studies on photosynthesis under different conditions and especially for studies on photosynthesis in absence of oxygen, since the luminous bacteria themselves may act as the means of absorbing oxygen. Statements in the literature as to the necessity of oxygen for photosynthesis are somewhat conflicting, but the general conclusion seems to be that oxygen is necessary (7, 8), different plants differing in their resistance to lack of oxygen. Willstatter and Stoll (10) found Pelargonium to be quite sensitive, while Cyclamen was very resistant to lack of oxygen. Despite the statements as to the necessity of oxygen for photosynthesis there is no theoretical reason why the first step in photosynthesis, the splitting of CO 2, which is the step detected by luminous bacteria, should fail in absence of oxygen. It would seem that further experiments in which it is certain that the oxygen is completely removed are desirable. In many experiments where gases are passed over cells to remove the oxygen, the gases may contain traces of oxygen which are easily detected by the luminescence of luminous bacteria. Harvey and Morrison (3) have found that light can just be detected in an emulsion of luminous bacteria 3 cm. thick when in equilibrium with 0.0053 mm. O2 or 0.0007 volume per cent, of oxygen. This will give some idea of the sensitivity of luminous bacteria as oxygen detectors. However, an emulsion of luminous bacteria becomes completely dark if undisturbed, and can reduce many dyes whose oxidation-reduction potential is well known. A knowledge of the potential of the dye allows us to calculate the pressure of oxygen with which equimolecular quantities of reduced and oxidized dye would be in equilibrium.