A two photocell assembly has been used to measure the light reflection of blood conducted by fibre optics. Fibre optic reflection spectra of oxygenated and reduced blood samples show an isobestic region around 850 nm. Non-specific variations of the light reflection from a mirror do not affect the ratio of detecting and compensating photocell outputs, whereas the ratio is not independent of variations in blood flow and haemoglobin concentration. The relation of log R640/R800 and the oxygen saturation is linear. The relation of R900/R800 and indocyanine green dye concentration is linear up to 20 mg/l.