Eisenin (C<sub>13</sub>H<sub>20</sub>N<sub>4</sub>O<sub>6</sub>) is a tripeptide isolated from the brown marine algae Eisenia bicyclis. It has immunological activity by augmenting natural cytotoxicity of peripheral blood lymphocytes (PBLs) in humans.
It can be synthesized by a tripeptide precursor known as y-methyl-l-glutamyl-y-methyl-l-glutamyl-l-alanine and an amide. This is done by the crystallization of the N-terminal of the y-methyl ester and the simultaneous amidation of the internal <small>L</small>-glutamic acid by ammonia.