Abstract: The apparatus is incorporated in a photometer utilized for measuring luminescence from a liquid sample containing adenosine triphosphate (ATP) and includes a cavity in a rotatable sample tube carrier inside a reaction chamber of the photometer and ports in the wall of the carrier communicating with the cavity, the ports being connected via tubing to a constant temperature liquid circulation system. The apparatus also includes a mechanism for rotating a sample tube received in the sample tube carrier when the sample tube carrier is rotated from a position outside the chamber to a position within the reaction chamber where a reagent can be inserted into the tube. The rotating mechanism provides for thorough stirring and mixing of the liquid contents of the sample tube and includes a rotatable journaled socket member at the bottom of the opening in the carrier for receiving the sample tube.