A systematic investigation of the preparation, characterization, and use of lithium phthalocyanine (LiPc) microcrystalline powder as a probe for electron paramagnetic resonance (EPR) oximetry applications has recently been initiated (Ilangovan et al, J. Phys. Chem. B 2000, 104, 4047-4059). Electrochemical preparation of LiPc under potentiostatic conditions (< 10 mG) yielded microcrystalline paramagnetic particles that showed extremely narrow (< 10 mG) and oxygen-dependent EPR line width measured at 9.82 GHz (Ilangovan et al. J. Phys. Chem, B 2000, 104, 9404-9410). In this report, we demonstrate the application of this material to measure oxygen concentration in a biochemical (xanthine/xanthine oxidase, X/XO), cellular (human polymorphonuclear neutrophils, PMNs), and in vivo (mice) biological systems. The microcrystalline particles showed high oxygen-sensitivity and stability in aqueous and physiological environments. The particles implanted in the gastrocnemius muscle of living mice were found to be stable enabling repetitive measurements of pO(2) for more than two weeks. Simultaneous measurements of oxygen consumption and oxygen radical generation from X/XO and activated PMNs systems were performed using a combination of the LiPc oximetry probe and DEPMPO spin-trap. The results demonstrated that the LiPc microcrystalline powder could provide accurate and repetitive measurements of oxygen in the medium of its dispersion or in any tissue region of pathophysiological interest.