We used an in vitro model consisting of a polycarbonate filter with pore diameters of 3 microns, through which red cells suspended in urine or buffer were pumped at a flow rate of 0.5 ml/min. pH and osmolality of the suspension were systematically changed within a range of pH 5-8 and 200-800 mosm/kg H2O. It was found that: (1) glomerular shapes could be produced by this filtration technique; (2) urine facilitated the production of glomerular shapes; (3) certain shapes occurred predominantly, at specific conditions of pH and osmolality and (4) hemolysis decreased with increasing osmolality or very low pH. These observations shed light on the mechanism by which glomerular red cell shapes are formed.