The force exerted on a trapped object depends on its size and shape, and on the refractive indices of the object and the surrounding medium. In order to use optical tweezers as a quantitative instrument, preliminary force calibration is necessary. Traditional way is the use of spherical polystyrene beads for the trapping force calibration.2,4,5,22 Isotonic buffer (phosphate-buffered saline: 10 mM disodium hydrogen phosphate, 1.76 mM potassium dihydrogen phosphate, 2.7 mM potassium chloride, 137 mM sodium chloride, osmolarity , pH 7.4) was prepared first. Osmolarity and pH of the buffer mimic the conditions of autologous plasma. Fresh blood (0.5 μ1) obtained by a fingertip needle prick was then diluted in 2 ml of phosphate buffer using a vacutainer containing EDTA. Carboxylated polystyrene beads, 3 μm in diameter, were added to the suspension ( particle per RBC) and then washed three times by centrifugation (Centrifuge 5417R, 3000 RPM, 25 °C, 5 min). After that, RBCs and polystyrene beads were suspended in 1 ml of autologous plasma. The coverslips for observation chamber were treated by serum bovine albumin (BSA) solution to prevent RBCs from sticking to the glass. One bead and one RBC were trapped by two independent laser beams with some distance between them and were moved to 10 μm above the lower coverslip. By varying the distance between the traps the particle and the cell were brought together until they contacted. The bead stuck irreversibly and nonspecifically to the RBC membrane in plasma within 1 to 2 min. The trap power for the RBC edge was set at 20 mW and the trap power for the bead varied in order to find the laser power conditions which would equalize the trapping force of the microbead and the RBC edge. The cell was then stretched by increasing the distance between the traps until the bead or the RBC edge slipped out from the traps. Escape trapping force for the bead and for the RBC edge was the same when the trap power for the bead was of , which was measured for five different samples. In order to obtain the value of this force, standard escape force method was used.3,4 Aqueous glycerol solution (15%wt.) was chosen as a calibration medium. Its index of refraction appeared to be 1.351 with Abbe number , which was measured using the Abbe refractometer. The same index of refraction value was obtained for plasma. As far as optical characteristics of both media were the same the trapping force of the calibration beads was expected to be the same in plasma and in glycerol. Provided by these characteristics, the same polystyrene particles as used in the previous measurements were suspended in glycerol solution and then trapped by optical tweezers at fixed height of above the surface of the lower coverslip. As the optical trap moved, solution exerted a viscous drag force on the trapped bead which is given as4,22, where is a viscous drag coefficient for a spherical particle with the radius in fluid at a height from the surface: Display Formula was taken to be (room temperature 25 °C). Viscous drag force was each time equal to the trapping force when the bead just escaped the trap. Provided by the value of the trap velocity corresponding to this case, the trapping force of the bead in our glycerol solution was obtained and appeared to be for the laser power of 17 mW. Such approach to calibrate the optical trapping force was appropriate in our case as far as the Reynolds number was (bead radius , glycerol solution density —for 15% glycerol solution). As a consequence, escape trapping force of the RBC edge in plasma was estimated to be .