The multiply charged ions produced by electrospray ionization of peptides of molecular masses up to 29 kDa have been successfully introduced into a Fourier transform mass spectrometer of unique capabilities for tandem mass spectrometry, large ion dissociation, and resolution. Electrospray ionization places an unusually high number of charges on a peptide yielding mass/charge (m/z) values of 600-1500; in this range at normal operating pressures (approximately 10(-9) torr; 1 torr = 133.3 Pa) Fourier-transform mass spectrometry resolving power is greater than 100,000. Although only 10(-7) torr pressure has been obtained with the initial interface, the resulting resolving power of 5000 makes possible the resolution of isotopic peaks of multiply charged ions. Mass measuring accuracies of a few daltons for molecular masses up to 17 kDa have also been achieved.