With the intension of exploiting latent information in a continuous spectrum we constructed an in vivo near infrared spectroscopy system including multicomponent analysis. The progress of acute rejection in orthotopically transplanted rat liver was monitored with this system. In the course of this study we found that nitrosylhemoglobin could be quantitated. The results revealed that increase of water content and decrease of the levels of oxyhemoglobin and oxidized form of cytochrome c oxidase,

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... orresponding to edema and circulatory failure in revascularized graft, took place at an advaced stage of rejection. On the other hand, rapid increase of nitrosylhemoglobin occured at the onset of the immune response, when any tissue damage was not brought about. Treatment with an immunosuppressant depressed these events. We also studied elongation of light path length due to multiple scattering in red blood cell suspensions and in a gerbil head under hypoxia.