Red-blood-cell fragments were produced when normal red blood-cells were forced, at arterial flow velocity, through a loose mesh of fibrin or other synthetic fibres. These fragments were morphologically indistinguishable from those which are found in the microangiopathic hæmolytic anæmia syndrome. Fragmentation resulted when membrane tears occurred in an arrested red blood-cell as a result of buffeting from rapidly flowing cells. The shape of the fragment formed depended on the position in which the red blood-cell was arrested and the site of membrane tears.