Summary and Conclusions: “Cold” or auto-agglutination was observed in the blood of a patient suffering from syphilitic liver cirrhosis with ascites and aortic regurgitation. The condition was not observed in the patient's two daughters. The “cold” or the auto-hemoagglutination took place only at (room) temperature, and the clumps broke down at 37°C. The reaction was reversible. The “cold” or auto-hemoagglutination was due to a substance, the “cold” or auto-hemoagglutinin, present in the serum. The serum was able to agglutinate at the low temperature the red blood corpuscles of any human blood group and rabbit's red blood cells as well. The serum contained no auto-hemolysin. The “cold” or auto-hemoagglutinin was present in the ascitic fluid in appreciable amount. As a result of absorption tests, the “cold” or auto-hemoagglutinin was found to consist of at least two substances, namely “cold auto-agglutinin” and “cold hetero-agglutinin.” The former which clumps the patient's own cells is perhaps different from the “cold isoagglutinin” which clumps the red blood cells of other individuals. The “cold agglutinin” was more active than the iso-agglutinin at 0°C., but less active at room temperature. The “cold agglutination” was found to be greatly accelerated by acetic acid and saturated solution of ammonium sulphate and inhibited by alkaline solutions. The mechanism of the “cold agglutination” is discussed. It is assumed that the “cold agglutinin” is precipitated by the “cold agglutinogen” upon the surface of the red cells, forming an unstable compound, thereby reducing or reversing their electrical charge. Normally the red blood cells are kept apart by the like charge. In this case, some of the cells have their charge reduced or reversed before others, so they attract their surrounding cells with opposite charge, forming clumps. As the compound formed by the “cold agglutinin” and “cold agglutinogen” combination is unstable, it will be broken down if heated to body temperature.