The Laboratory Proficiency Testing Program has successfully operated mandatory proficiency testing for 13 years in Ontario, Canada, using committees of volunteer peers. This is an alternative to gubernatorial proficiency testing. Using white blood cell (WBC) count determinations as a focus, we present our results. Ninety-six percent of participants attained WBC count results within +/- 10% of an all-methods mean and 77% attained results within +/- 5%. Ninety-six percent displayed satisfactory precision. Details of the program, ie, testing material, computer analysis, reference results, and scope of committee action, are discussed. In 1975, clinical expectations for WBC count determinations greatly exceeded most laboratories' capabilities. By 1987, participants were able to produce reliable, clinically useful WBC count results. The paramount reason for this improvement is the transition from manual to automated counting methods resulting from the stimulus of proficiency testing and the concomitant education.