The biological variation of the blood concentration and number fraction of the different kinds of leukocytes have been estimated, over a one-year period in a group of 19 women and 20 men, apparently healthy volunteers. All measurements have been performed with a Toa E-5000 haematological analyser. The medians of the within-subject biological coefficients of variation, separated by sex when significant differences exist, are the following for concentrations: 9.4% in men and 15.4% in women for the leukocytes, 16.9% in men and 26.9% in women for the neutrophils, 12.3% for the lymphocytes, 14.9 for the basophils+eosinophils+monocytes (as measured by Toa E-5000); and for number fraction are the following: 8.6% for neutrophils, 0% in men and 7.2% in women for lymphocytes, and 16.7 for basophils+eosinophils+monocytes (as measured by Toa E-5000). With these data the index of individuality, the critical difference for significant change detection and the desirable imprecision have been calculated.