We developed an in vitro model to produce urinary dysmorphic RBCs and to investigate the influence of pH and osmolality. The pH and osmolality of the RBCs suspension was made to change systematically within a range of pH 5-8 and 200-800 mOsm/kg, and at a flow rate of 0.5 ml/min through a polycarbonate filter (Calif., USA) with pore diameter of 3 mu. Then the samples were examined with phase-contrast microscopy. Our data showed that: (1) The formation of glomerular RBCs depends on their passage through a narrow pore and the presence of certain suspension, which is also present in glomerulonephritis. Suspension of RBCs in urine with the same osmolality and pH, but without filtration did not produce glomerular shapes. (2) Urine facilitated the production of glomerular shapes. Filtration of RBCs suspension in physiological buffer instead of urine produce less dysmorphic shapes than filtration in urine of various pH and osmolality. (3) Certain types of urinary RBCs shapes occurring predominantly at specific conditions of pH and osmolality (< 400 mOsm/kg) produced glomerular shapes most frequently, e.g. doughnut-like RBCs were produced by urine with pH 7. Our data suggested that the mechanism of glomerular RBCs formation might result from the passage of RBCs through narrow defects in the ruptured glomerular capillary wall and a chemical injury to the RBCs in the tubular lumen induced by urinary constituents.