The Hemox-Analyzer (TCS, Medical Products Division, Southampton, PA) is an automatic system for determining the oxyhemoglobin dissociation curve (ODC) and P50 values. The ODC is recorded during deoxygenation with nitrogen gas and plotted on graph paper; the oxygen tension is detected by a Clark electrode, while the oxyhemoglobin fraction (% HbO2) is evaluated by a dual-wavelength spectrophotometer. Even though this instrument has been commercially available for more than 20 years, its performance characteristics have been assessed. We evaluated the performance characteristics of the Hemox-Analyzer. P50 was tested in 28 healthy volunteers, in 16 anemic and in 9 polycythemic patients. To test its precision we evaluated both inter-and intra-assay variability. The system shows good precision: standard deviation was 0.39 for assays in duplicate, CV= 1.9% for intra-assay and CV= 3.0% for inter-assay measurements. The mean P50 values were 25.2+/-1.5 mmHg in normal volunteers and 27.3+/-1.4 mmHg in anemic patients. The Hemox-Analyzer is a simple, quick and reliable instrument for recording the oxyhemoglobin dissociation curve. Both the P50 value and observation of the fine structure of the curve can furnish information about the delivery of oxygen to tissues.