Two samples of a manual 3 L calibrating syringe which displays an electronically calculated FEF25-75% were evaluated to determine its suitability as a flow calibrator were evaluated. Room air was discharged into a manual spirometer system known to be accurate. The calibrator-determined values correlated very closely with the spirometer values over an FEF25-75% range of 0.4 to 9 L/sec. The differences between calibrator and spirometer FEF25-75% values were small (mean +/- 1 percent, greatest 3.7 percent) and of little importance clinically. This portable, simple to operate calibrating syringe provides accurate FEF25-75% and volume values. By adding flow calibrating capability to a recommended standard volume calibrating syringe, it will facilitate the routine calibration of spirometers in the laboratory and in the field. Since it uses displaced air, it can be used at altitude and with flow measuring instruments without the correction required for calibrators using CO2 cartridges.