This paper deals with measurement of the hemoglobin concentration in individual human red cells, the concentration being measured as a function of the optical density, at 5461 A, of the central part of the cell in its spherical form. A number of technical difficulties, principally concerned with the effects of focus, have been met with, and these are described in detail. The results show that the coefficient of variation of the hemoglobin concentration is relatively small (about 5.5 per cent) in the case of fresh human red cells in ACD solution, that the coefficient of variation is almost doubled when the red cells have been stored for 6 weeks at 4°C. or when heparin is used as an anticoagulant instead of ACD solution, but that the average hemoglobin concentration is substantially the same in all three cases. The increase in the coefficient of variation found when the red cells have been stored or when heparin is used is probably due to volume changes, some cells shrinking while others swell.

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