AbstractA radioimmunoassay (RIA) for serum thymic factor (facteur thymique serique) was established. This assay could detect<0.5pg of serum thymic factor without the interference by other thymic hormones, including thymopoietin ii and thymosin α1. Both zinc‐free and zinc‐bound serum thymic factor showed a similar standard curve, indicating that this RIA could detect both forms of serum thymic factor in the same way. We observed that two procedures were required prior to the measurement in order to measure serum thymic factor content in plasmas; the first, the extraction procedure to remove substances interfering with the assay nonspecifically; the second, the storage of the sample with ethylenediamine tetraacetic acid dissodium salt to inhibit the proteases in the plasma to degrade serum thymic factor. By employing this assay system, we observed that cord blood plasmas contained significantly higher level of serum thymic factor than did adult blood pla