Peptide derivatives of formula ##STR1## wherein R.sup.1 represents a substituted amino group which is capable of being split off enzymatically with formation of a colored or fluorescent compound R.sup.1 -H. The said peptide derivatives are used for quantitatively assaying the enzyme C.sub.1 -esterase. The assaying is carried out by reacting a medium which contains C.sub.1 -esterase or in which the latter is formed or consumed with a peptide derivative as defined above and measuring photometrically, spectrophotometrically, fluorescence-spectrophotometrically or electrochemically the quantity of split product R.sup.1 -H released per time unit by the catalytic hydrolytic action of the said enzyme on the peptide derivative.