Three different species of red blood cells have been tested by a new image analysis method to determine cell-cell adhesion with a high level of precision. Various literature sources have suggested that horse and rat erythrocytes adhere to form aggregates more readily than human red cells but this paper provides the first accurate quantification of this phenomenon. Addition of surface active agents such as glutaraldehyde. papain and fibronectin was also tested in order to measure the effects on cell-to-cell adhesion. Glutaraldehyde reduced adhesion whereas both papain and fibronectin increased it.