Because of limitations associated with commercial blood gas quality controls and tonometry of stored blood, fresh heparinized blood was used to compare PO2 and PCO2 performance of ten blood gas analyzers. Function of nine gas mixer/tonometer systems was evaluated. These were used to create blood samples with target values for PO2 and PCO2. All ten analyzers had high precision; this magnified small differences between observed results and target values. Grand mean results from all ten analyzers were within 0.8 mm Hg of the target for PO2 of 40.0 and 100.0 and for PCO2 of 20.0 and 70.0 mm Hg. Eight automated blood gas analyzers gave clinically indistinguishable results for PO2 and PCO2 that were sufficiently accurate for clinical purposes. Also reported is the practicality of using tonometry of fresh heparinized blood in a per shift quality control program for PO2 and PCO2.