The total dry mass of human erythrocytes was determined by both interference microscopy and x-ray microradiography. The determination of mass per unit area, and calculation of total dry mass per cell were simplified by changing the shape of the cells to spheres which were then flattened to discs of constant thickness when smeared on glass slides for measurement of fixed cells by interferometry, and to oblate spheroids when smeared on parlodion-coated slides for measurement of fixed cells by x-ray absorption. From x-ray measurements of 100 smeared and alcohol-fixed cells a mean dry mass per cell of 33.7 x 10-12 g was obtained. Interference measurements of 100 fresh cells suspended in isotonic saline gave a mean value of 32.4 x 10-12 g while interference measurement of 100 smeared and alcohol-fixed cells gave a mean value of 30.8 x 10-12 g. The first two values compare well with a mean corpuscular hemoglobin of 31.2 x 10-12 g, obtained from determinations of erythrocyte count and hemoglobin, since 95 per cent of the dry mass of the cell is hemoglobin. The difference in interference values between the fixed and fresh cells is possibly due to a difference between the specific refractive increment of alcohol-denatured hemoglobin and that of the unmodified substance. The value for the latter was used since that of the former is unknown.

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