Picomole samples of the linear peptide gramicidin D and cyclic peptide gramicidin S are shown to be impure by the laser-desorption formation of multiple groups of molecular adduct peaks by using Fourier-transform mass spectrometry. Selective excitation of the molecular peaks of the major sample component followed by collisionally activated dissociation provides complete sequence information for the cyclic decapeptide and for 12 of the 15 amino acids of the linear peptide. This instrumentation shows striking advantages in sensitivity, resolution, and mass accuracy in comparison to tandem mass spectrometers used previously.