Oxygen dependent quenching of phosphorescence has been used to measure the oxygenation of tissue in mice, including the differences between normal tissue and that of a murine tumor. Approximately 0.3 mg of the phosphorescence oxygen probe, Green 2W, was injected into the tail vein of tumor bearing mice. The mice were immobilized using an anesthetic cocktail and illuminated with flashes (< 4 microseconds t1/2) of light of 636 +/- 15 nm. The emitted phosphorescence (790 nm max.) was measured using an imaging phosphorimeter with an intensified CCD camera, an instrument which provides two dimensional digital maps of oxygen pressure. Both the illumination light and the phosphorescence were in the near infra red region of the spectrum, where skin and tissue have little absorption. The light can therefore readily pass through the skin and centimeter thickness of tissue. Mice are sufficiently small that the oxygen pressure maps could be obtained by illuminating from either the same or the opposite side as the camera (and tumor). The tumors were observed as regions with oxygen pressures substantially below those of the surrounding normal tissue. Thus, it is possible to non-invasively detect these tumors and to monitor their internal oxygen pressure in real time and through cm of tissue.