Prothymosin α, an immunoactive polypeptide of 12 kDa, has been isolated from porcine thymus, spleen, lung and kidney. It lacks aromatic and sulfur‐containing amino acids and has a high content of glutamic and aspartic acids. Tryptic digestion of porcine thymus prothymosin α yielded peptides which on separation, amino acid analysis and alignment with the known sequence of prothymosin α from rat and man showed that the amino terminal portion of the molecule is conserved and the few differences present are confined to the car☐y terminal.