In order to evaluate the Cobas Argos 5 Diff, a five-part differential haematological analyser, we performed a comparative study using our current analyser, the Toa E-5000, and the manual leukocyte differential. Samples (n = 593) were collected from various departments in our hospital and were tested on both Cobas Argos 5 Diff and Toa E-5000. Regression lines were calculated for all standard haematological quantities. A group of 100 samples, including 50 negative (absence of flags) and 50 positive (presence of flags), on the Cobas Argos 5 Diff were randomly selected from the above-mentioned collection. All these samples had been checked under the microscope by two persons counting 200 cells each. The comparison of standard haematological quantities between Cobas Argos 5 Diff and the Toa E-5000 shows good correlation: Red Blood Cells r = 0.993; White Blood Cells r = 0.998; Haemoglobin r = 0.989; Haematocrit r = 0.982; Thrombocytes r = 0.988; Lymphocytes (%) r = 0.983; Neutrophils (%) r = 0.959. The correlation between Cobas Argos 5 Diff and the manual leukocyte differential shows good correlation for lymphocytes and neutrophils, with acceptable correlation for eosinophils. The correlation for basophils and monocytes was less acceptable, especially for normal samples. We conclude that the efficiency of the Cobas Argos 5 Diff is good (93%) and that it is a suitable haematological cell analyser.