Abstract: A high sensitive and stable blood leak detector suitable for use with artificial kidneys. The detector has an optical portion comprising a measurement cell through which dialysis solution under test passes, an alternately energized pair of light-emitting diodes positioned on one side of the cell, and a photo sensitive detector positioned at the opposite side of the cell and close thereto. One light-emitting diode emits radiation at longer wavelengths, and the other at shorter wavelengths. Both the diodes are adjusted to produce photo detecting signals at the same level when no blood exists in the cell. The photo detecting signals at the photosensitive detector are detected and analyzed to give signals deflected in one direction in proportion to the degree of blood leak when it has occurred. When bubbles are detected, they give signals in the opposite direction, based on the fact that blood absorbs light mainly at shorter wavelengths while bubbles scatter light mainly at longer wavelengths.