The thesis deals with two objectives of protein analytics employing modern complementary mass spectrometric techniques. Subject of the first part is the release of bioactive peptides during the digestion of food proteins. Hence, the milk protein β-casein was hydrolysed under gastric conditions and its degradation products were characterised by means of electrospray or MALDI mass spectrometry. 138 different peptides were identified and with these a complete sequence coverage was obtained. The results provide insights into the time course of the digestion, the preferred cleavage sites of the protease pepsin, and the fate of bioactive β-casein peptides. Moreover, an amino acid variation observed in position 93 proves the existence of an additional genetic variant of bovine β-casein which has not yet been defined. The second part addresses the investigation of the primary structure of elastin, an insoluble fibre protein. Elastin like other structure proteins is a component of the extracellular matrix and provides elasticity to the tissue of many organs. It was demonstrated that elastin isolated from human skin can be effectively digested by different proteases of broad specificity. The used enzymes elastase, thermitase, and pepsin predominantly cleaved C-terminal to four of the five major amino acids of elastin. Overall, 254 peptide sequences were determined resulting in a sequence coverage of 74 % with respect to the precursor tropoelastin. The results facilitate conclusions about the alternative splice variant of human skin elastin on the protein level. For the first time, the extent and location of proline hydroxylation in human skin elastin were determined.