We investigated a flow cytometric method with monoclonal antibody CD42a as a potential reference method for platelet enumeration by blood count analysers. Using peripheral blood samples from 25 healthy individuals we obtained significant (P < 0.0001) correlations between the results obtained by a FACScan method and similar cell counters (TOA Medical Electronics, Kobe, Japan): r = 0.960 (FACScan vs SSF), r = 0.958 (FACScan vs SE-9000A,B), r = 0.949 (FACScan vs K-4500A) and r = 0.954 (FACScan vs K-4500B). This flow cytometric method for counting platelets using the monoclonal antibody CD42a can be used to check the calibration of the platelet count by blood cell analysers.