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The Travenol Hem-O-Scan and the Instrumentation Laboratory (IL) system for determination of the oxygen dissociation curve and P50 values have been evaluated. Using the Hem-O-Scan, an oxygen dissociation curve and P50 value may be obtained on 2 microliter of blood in 40 min, though the pH is not measured. In the IL system, approximately 3 ml of blood is required to establish four points on the oxygen dissociation curve in 80 min, and pH data are available. Both systems give reproducible results for P50, giving standard deviations of 0.786 mm Hg for the Hem-O-Scan and 0.949 mm Hg for the IL system, compared with 1.740 mm Hg for the manual system. Good agreement was found between the manual method and Hem-O-Scan (t = 0.363, degrees of freedom = 19, 0.8 greater than P greater than 0.7; r = 0.892, P less than 0.001). There was also satisfactory agreement between the IL system and the Hem-O-Scan (t = 0.370, degrees of freedom = 20, 0.8 greater than P greater than 0.7; r = 0.760, P less than 0.001). P50 values obtained by the manual and by both automated technics agree well with published values.