The formation of platelet aggregates which embolize to the peripheral circulation has previously been noted as a significant deleterious effect resulting from both intra- and extracorporeal artificial circulatory devices. Utilizing the stagnation point flow experiment, which permits visualization during flow of aggregate formation on first contact of blood with an artificial surface, the formation of freely floating aggregates has been observed in separated flow regions. Embolization from the separated flow has also been noted. Comparison of observed growth rates with a hydrodynamic model suggests that sufficient activation has occurred within the separated region so that platelets stick on virtually every collision. Some criteria are also suggested which correlate with the flow conditions affecting aggregate formation. At high flow rates, where freely floating aggregates do not form, significant surface thrombi are found.