Abstract: A calibration apparatus for checking the accuracy of DLco testing machines includes a pair of syringes mounted side by side in a frame and joined by a valve controlled gas passageway to a coupling adapted to mate with the testing machine. The first syringe receives gas from the testing machine and the second syringe has a chamber from which gas of a known concentration is returned to the testing machine. The chamber of the second syringe is flow connected by a valve to a source of test gas. The gas passageway is constructed in such a manner to reduce dead air space and to allow purging with test gas to eliminate contamination with ambient air or gas of previous tests.