The choice of an automated differential leukocytes counting system depends on a number of criteria including reliability, speed an cost/effectiveness ratio. We have compared the performance of three commercially available instruments, the Hematrak 360 (Geometric data), the ADC 500 (Abbott) and the DIFF 3 (Coultronics). We have voluntarily confined this study to pattern recognition systems and have not investigated other systems based on cell volume determination and cytochemical reactions (Hemalog D, Technicon). Comparison of a new system with an established automated system simplifies the task considerably, provided that the quality and performance of the reference system have been previously well-established, as has been done in our laboratory with the LARC which has been evaluated comparatively to the traditional method (5). A final difficulty is that the conclusions drawn from this type of study cannot be considered as definitive insofar as changes and improvements are continually being proposed for all systems which are still going through a mutation stage as we noticed in the course of this evaluation.