The increased inhibiting action of serum with age on the growth of fibroblasts has been shown to be due to changes in both the lipoids and the proteins.

Although the serum protein is less inhibiting than the serum lipoid, the rate at which their inhibiting action augments with age is approximately the same.

The change in the serum protein with age is due to the disappearance of a small quantity of growth-stimulating substance, and also to an increase in concentration of inhibiting protein. The concentration of all protein fractions becomes larger as age advances.

The greater inhibiting action of the lipoid is associated with a higher concentration of total lipoid and lecithin, and a smaller content of cholesterol as the animal grows older.

The hypothesis is suggested that the inhibiting action of the serum is associated with its antienzymatic action.

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