[Modification of the antigenicity of the serum thymic factor (thymulin) by zinc]

We have previously reported that the serum thymic factor (FTS) exists under two forms, one deprived of zinc, biologically inactive and another one containing zinc, biologically active for which we have proposed the name of thymulin (FTS-Zn). A new iodinated tracer (125I FTS-Zn) has been prepared by fixing zinc on synthetic FTS. This tracer is recognized by a Rabbit anti-FTS antiserum at a 1/20,000 dilution. Normal Mouse sera displace the binding of FTS-Zn to the antiserum (40-60% inhibition) whereas thymectomized mouse sera have no effect. These results should allow the establishment of a simple radioimmunoassay for thymulin (FTS-Zn) present in biological fluids.