The morphology of red blood cell (RBC) aggregates was studied by direct visualization of RBC aggregation at different flow conditions in a computerized image analyzer. The aggregate morphology is expressed by an Aggregate Shape Parameter (ASP), defined as the ratio of the aggregate projected area t o its square perimeter. Aggregation was induced by either dextran-70 (mw 70,000) or dextran-500 (mw 500,000), and compared to that in plasma. It was found that the aggregate morphology is a characteristic of the aggregating agent—in dextran-500, the RBC form rouleau aggregates as in plasma, while in dextran-70, they form clusters. In each system, while maintaining the overall typical morphology, the ASP decreases (ie, the aggregate becomes longer) as the aggregate size is increased. The distribution of the ASP as a function of the aggregate size remains unchanged when the aggregate size is changed by modulation of the dextran concentration or the shear stress. Stretching of a rouleau aggregate by application of shear stress is reflected by a corresponding change in the ASP. It is suggested that the ASP is a characteristic of intercellular interactions. A theoretical model is proposed for evaluation of the deviation of aggregate shape from that of rouleau structure.