AbstractThe determination of leukocyte concentration in human blood depending on the detection of oxidized o‐dianisidine in acidic solution is studied. The oxidation of o‐dianisidine was carried out by peroxidase enzymes found in leukocytes. The reaction was stopped by the addition of 4N H2SO4 to the solution, and a very stable, colored o‐dianisidine derivative was obtained. The calibration graph was plotted with the recorded absorbance values at 400 nm assigned to the y‐axis, and leukocyte counts in 1‐mL blood samples to the x‐axis. The equation of the calibration graph was y=0.0025x+0.0904, with a correlation coefficient of R=0.994. The coefficient of variation and P‐value of the method were 4.00% and 0.05%, respectively. J. Clin. Lab. Anal. 16:233–236, 2002. © 2002 Wiley‐Liss, Inc.