Radioactive iron was administered to three normal dogs, two of which had previously been bled, in order to tag a group of erythrocytes of approximately known age.

The osmotic fragility of the newly formed tagged cells was significantly greater than that of the general cell population during the first few days after injection of the iron, while the mechanical fragility of the young cells was less than that of the general red cell population. As the cells aged and approached the end of their life span, their susceptibility to destruction by trauma inflicted by rolling glass beads exceeded that of the general cell population. The osmotic behavior of the old cells was not distinctive.

The increased mechanical fragility of senescent cells suggests that the life span of erythrocytes may be limited at least in part by changes within the cell which render it more susceptible to destruction by mechanical wear and tear in the circulation. It is emphasized, however, that the trauma produced by rolling glass beads may be quite unlike that inflicted upon red cells in vivo.

A decrease in circulating radioactive iron was observed in each experiment soon after the mechanical fragility of the tagged cells began to exceed that of the total cell population. The lowest point on the curve representing circulating radioiron was noted at 119, 119, and 122 days respectively after injection of iron in the three experiments. Estimates of the life span of dog erythrocytes obtained in this way agree with those provided by other methods.

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