Evidence of the presence in normal serum of a carrier of the serum thymic factor (FTS).

Chromatography of normal human or mouse serum on Sephadex G-150 revealed the presence of two peaks of activity in the rosette assay used for the characterization of the serum thymic factor (FTS). One peak corresponded to the elution volume of FTS (mol. wt. 867), the other to molecules with mol. wts. of 40 000-60 000. Both peaks were absent in serum of thymectomized (Tx) mice but could be induced in such Tx mice by injection of synthetic FTS. Study of the biological half-life of both peaks (assessed by the rosette assay) and demonstration of their specific retention on anti-FTS immunosorbent indicate, altogether with direct binding experiments, that FTS is transported in serum by a molecule with a mol. wt. of the order of that of albumin or prealbumin. It is apparent that FTS bound to this molecule is responsible for the biological FTS-like activity associated with large mol. wt. fractions of normal serum.