It has been suggested that freshly obtained red cells fail to agglutinate in the presence of antibody, and that the same cells left for several days at 4C in anticoagulant will usually be agglutinated by the same antibody, erythrocyte stabilization being responsible for this differential effectiveness of agglutination. Since supportive documentation for this assumption is not available, and since the concept of differential effectiveness of agglutination as a function of red cell age may be of great significance to the sensitivity of laboratory procedures which take advantage of antigen/antibody reactions, the above hypothesis was investigated using a cold agglutination procedure as a test system. Cold hemagglutination was chosen for reasons of expediency. Results of that investigation, possible explanations for the original hypothesis, and suggestions for further testing are presented here.