Abstract: A device for automatically analyzing samples by a colorimetric procedure may be particularly used for blood analysis. A reaction ring bearing at its periphery an array of transparent reaction cups is stationary. An optical fiber is mounted on a rotating disk, concentric with the reaction ring and downwardly oriented with respect to the top of a cup, the optical fiber having one end at a central location where it receives light from a stationary light source and its other end opposite a bottom of a reaction cup in alignment with a measuring photomultiplier.