Two new peptides, designated thymosin beta 8 and thymosin beta 9, respectively, have been isolated and their amino acid sequences established. Thymosin beta 8, isolated from calf thymus fraction 5, has a mass of 4518 daltons and contains 39 amino acid residues, of which 31 are identical to the corresponding amino acid residues in thymosin beta 4 isolated from the same source. The NH2 terminus of thymosin beta 8 is acetylalanine, compared with acetylserine in thymosin beta 4. Thymosin beta 9, isolated from fresh-frozen calf thymus by a procedure that minimizes proteolysis, is identical to thymosin beta 8 except for the presence of an additional dipeptide, -Ala-LysOH, at the COOH terminus. It has a mass of 4717 daltons and 32 of its 41 amino acids are identical to those of thymosin beta 4. The similarity in structures of thymosin beta 4 and thymosin beta 9 suggests that they may have related functions.