Abstract: The present invention is directed to the protein human splenin (hsP), substantially free of material associated with the protein in its natural environment in the human spleen. In particular, the present invention relates to such isolated or synthesized human splenin having the amino acid residue sequence:-GLY-LEU-PRO-LYS-GLU-VAL-PRO-ALA-VAL-LEU-THR-LYS-GLN-LYS -LEU-LYS-SER-GLU-LEU-VAL-ALA-ASN-ASN-VAL-THR-LEU-PRO-ALA -GLY-GLU-MET-ARG-LYS-ALA-VAL-TYR-VAL-GLU-LEU-TYR-LEU-GLY -SER-LEU-THR-ALA-GLU-HIS-.Additionally, the present invention relates to a therapeutic composition of matter comprising a therapeutically effective amount of the hsP having the above sequence and a pharmaceutically acceptible carrier.