Stroma-free hemoglobin (SFH) solution is an acellular oxygen carrier under development as a temporary red blood cell (rbc) substitute. SFH will be of value if it contributes to oxygen loading (delivery) and unloading (consumption). If there is no interaction between red cell hemoglobin and SFH, then the composite oxygen dissociation curve for whole blood (WB) can be written as: SWB= r SSFH+ (1 - r) Srhc, where S is the fractional saturation, and r is the SFH/WB hemoglobin ratio. This composite whole-blood curve was compared with the calculated curve, and the root mean square difference was 0.9%. Direct and indirect methods of measuring oxygen concentration ([O2]) of whole blood or plasma were compared. The regression line was [O2]indirect.= 0.995 [O2]direct+ 0.07 (r = 0.995, N = 16,p< .01). The adequacy of anaerobic separation of plasma was evaluated by measuring the changes in Po2and Pco2. The Po2increased by less than 2 torr and the Pco2decreased by less than 1 torr during in vitro tests. Both the direct and indirect methodology yielded consistent [02] values for each carrier.