The possible role of white blood cells in tissue ischaemia has attracted recent interest. White blood cells can block narrow vessels, particularly if perfusion pressure is reduced or if the cells become activated. To investigate the role of white blood cells in ischaemia microfiltration techniques were used to measure the flow properties of these cells in patients with severe leg ischaemia before and after amputation. Compared with controls white blood cells from patients showed impaired ability to flow through 8 micron and 5 micron pore filters. This applied to fractionated granulocytes and mononuclear cells as well as to unfractionated mixed white blood cells. White blood cells from blood draining the ischaemic leg had worse filterability than those from arm blood of the same patients. After amputation of the ischaemic leg there was definite improvement, the flow properties of the cells being no longer significantly different from controls. These abnormalities detected in white blood cells probably reflect activation of the cells by factors released in the ischaemic tissue. As activation alters the mechanical and adhesive properties of white blood cells, a vicious cycle of microcirculatory trapping at low perfusion pressure, activation, tissue damage, and further activation and trapping might contribute to the progressive worsening of tissue ischaemia.