An assessment of the Coulter model S automatic blood counter has been carried out. The standard deviations for the haemoglobin concentration, haematocrit, red cell count, and white cell count are, respectively, ± 0·15 g/100 ml, ± 0·45%, ± 0·04 m/c mm, and ± 0·47 thous/c mm. These results are clearly more accurate than careful manual estimates, performed for comparison on the same samples. Details of the comparisons are presented. A comparison is also made with routine daily estimations.