Contribution of the modern mass spectrometry to the structure determination of cyclic peptides is explained with compounds belonging to a pharmaceutically important group of cyclosporins. In addition to the determination of molecular weight and elemental composition of molecule, also the presence of individual amino acids may be estimated. Their sequence can be derived from the daughter ion mass spectra of the protonated molecule obtained in a tandem experiment. Comparison of the characteristic fragmentation of immonium ions in the spectra of peptides and free amino acids makes possible to distinguish even isobaric amino acids one from another. Examples of use of the tandem mass spectrometry in the analysis of mixtures are presented. Coupling of high-performance liquid chromatography and mass spectrometry utilizing fast-atom bombardment, electrospray, and atmospheric-pressure chemical ionization, is discussed. The quantitative analysis of cyclosporins in biological samples using laser desorption or fast-ion bombardment with the time-of-flight ion analysis surpasses immunologic assays in selectivity and fast performance.