Abstract: The subject of this present invention is tripeptides of N.sup.3 -4-methoxyfumaryl-L-2,3-diaminopropanoic acid of the general formula ##STR1## where R is a hydrogen atom when R.sub.1 is the dipeptide residue containing the residue of alanine, methonine, valine, leucine or norvaline, or R and R.sub.1 have the same meaning and are the residue of monoaminomonocarboxylic aminoacid, such as those of alanine, methionine, valine, leucine, or norvaline, or R is the dipeptide residue containing the residue of alanine, methionine, valine, leucine, norvaline, lysine, ornithine, sarcosine, 2,4-diaminobutanoic acid and 2,3-diaminopropanoic acid, and R.sub.1 is a hydroxide group, and the method of their obtaining. The method of obtaining involves converting N.sup.2 -tetrbutoxycarbonyl, N.sup.3 -4-methoxyfumaroyl-L-2,3-diaminopropanoic acid into an active ester which is used for acylating a dipeptide or N.sup.2 -tetr-butoxycarbonyl, N.sup.