Matrix-assisted laser desorption/ionization (MALDI) has been used with an external ion source Fourier-transform mass spectrometer to obtain the highest mass resolution ever, to our knowledge, demonstrated for laser-produced ions (m/delta m = 1,100,000 for [Arg8]vasopressin, 228,000 for melittin, and 90,000 for bovine insulin). The peaks in the isotope cluster for bovine insulin are fully resolved, and the mass measurement accuracy is an order of magnitude better than can be achieved with time-of-flight mass spectrometry. With the method described here, analyte is applied to a sample probe and mixed with a solution containing a matrix material (2,5-dihydroxybenzoic acid) that strongly absorbs ultraviolet light. Upon irradiation with a pulse from an excimer laser (353 nm, 2 mJ), a large number of intact protonated molecular ions are produced. The ions are focused by a 117-cm-long quadrupole ion guide and injected into an ion cyclotron resonance analyzer cell located inside the bore of a 6.5-T superconducting magnet. A pulse of argon buffer gas cools the ions prior to detection. One of the principal advantages of an external ion source Fourier-transform mass spectrometer is that the ion formation and ion detection processes are separated and can be independently optimized.