Blood from carriers of hemoglobin Osler (Hb Osler) had almost the same oxygen affinity as that of carriers of Hb McKees Rocks (Hb MR) (P50 10- 11 mm Hg), but Hb concentrations were higher in male carriers of the former (21.6 versus 17.2 g/dl). Two carriers of each Hb were studied to compare their adaptations to altered oxygen affinity and their responses to phlebotomy. All four were healthy, and all excreted normal amounts of erythropoietin. Carriers of Hb MR had somewhat lower mixed venous pO2 than carriers of Hb Osler. There was no suggestion that phlebotomy impaired ability to exercise in either group of patients.