The method of assaying rennin described by Berridge(1) requires the formation under reproducible conditions of a thin flowing film of milk. Although the skill needed to keep the film flowing at the optimum rate as the clotting point is approached is readily acquired, an automatic method has the advantage of complete reproducibility. A suitable film can be maintained if 10 ml. of milk are placed in a boiling tube of 2·2 cm. internal diameter and the tube, with its axis inclined at about 30° to the horizontal, rotated at 3·0 r.p.m. In the apparatus now used, the boiling tube is mounted on a rubber bung, which in turn is mounted concentrically on a spindle connected through Meccano gears to a 'Sangamo' electric clock motor. A special mechanical coupling allows the bung to be rotated by hand one revolution forwards, thus allowing 20 sec. for fitting or releasing the boiling tube before the drive is again taken up.