The automation of platelet counting is essential in laboratories performing a large number of procedures with high precision. Maintaining this precision and establishing accuracy require an understanding of the special problems of counting platelets, including: (1) the large dynamic range of the measurement, (2) the variable size and aggregability of platelets, and (3) the specific requirements of a quality-control method. In evaluating these problems, we designed experiments that measured the linearity and precision of two types of platelet counters, light scattering and electronic aperture impedance, and that evaluated the suitability of commercially available reference materials. The results show that the instrumentation is excellent and the reference materials are good; such that, given a well-planned quality-control method as presented here, the automated platelet count gives a rapid result with precision and accuracy.