Abstract: The invention relates to, novel peptide derivatives with the formula: R.sub.1 -A.sub.1 -A.sub.2 -A.sub.3 -A.sub.4 -R.sub.2 or its salt, wherein R.sub.1 =H or a protective group, A.sub.1 =H, Ile, Leu or Val, A.sub.2 =Glu, Asp, Ser, Thr; A.sub.3 =Gly or Glyc; A.sub.4 =Arg or Lys; R.sub.2 =4-nitroaniline with the proviso that A.sub.3 is Gly when A.sub.4 is Lys and that A.sub.3 is Glyc when A.sub.4 is Arg. The invention also discloses the process for the preparation of the peptide derivatives, the method for the determination of the bacterial endotoxins by the use of the invented peptide derivatives and the use of these derivatives for determination of bacterial endotoxins.