Thymosin α₁ and thymosin β₄ were first isolated from thymosin fr. 5 and have demonstrated biological activities on the immune system. They are chemically distinct and differ in their immunological activity profiles. The levels of thymosin α₁ and thymosin β₄ were assessed by radioimmunoassay in the same serum samples. Normal thymosin α₁ levels were 670 ± 163 pg/ml for males and 652 ± 162 pg/ml for females. Normal thymosin β₄ levels were 974 ± 400 ng/ml for males and 889 ± 345 ng/ml for females. No correlation between the levels of the peptides in serum from normal donors was observed. Although many samples of serum from neonates (cord blood), homosexuals and AIDS patients had elevated levels of one or both peptides, no correlation between the two peptides was found. Of potential significance is the observation that while thymosin α₁ and β₄ are elevated in many individuals with AIDS (57 and 48% respectively), the individuals with AIDS related immune dysfunctions had predominantly elevated thymosin α₁ (54 vs 15%).

These studies suggest that serum levels of the two peptides are modulated separately and that both are of potential value in defining the risk of individuals for developing AIDS.