Abstract: A method and apparatus monitor calibration of a hematology parameter measurement system. A baseline control value measured on an unassayed sample is entered and stored in a register in a calculator. Sample parameter measurements are entered in the calculator, and a prior art "moving averages" method is used to calculate a "moving averages" value indicative of calibration drift. After a preselected time or number of sample measurements, the same unassayed sample is again measured. The measurement is entered. Calibration status is monitored by comparison of the sample measurements and by comparison of the moving averages value to a preselected threshold level.