The human U1 snRNP (small nuclear ribonucleoprotein), which is a part of the spliceosome, consists of U1 snRNA and ten different proteins: seven Sm proteins B/B', D1, D2, D3, E, F, and G and the three U1-specific proteins U1-70 K, U1-A, U1-C. To determine the stoichiometry of all ten proteins, the complex was denatured, digested completely with an endoproteinase and labeled with an amine-specific tag. Corresponding peptides were synthesized and labeled with the same tag containing heavier isotopes. The digest was then spiked with defined amounts of the synthetic peptides, and the resulting isotopic peptide pairs were analyzed quantitatively by mass spectrometry. The mass spectra provided information about the absolute amount of each component in the starting protein mixture. The use of the isotope-coded, amine-specific reagents propionyl-N-oxysuccinimide and nicotinoyl-N-oxysuccinimide was evaluated for stoichiometry determination; the nicotinoyl reagent was found to be advantageous because of its greater mass spectrometric sensitivity. Absolute quantities of all ten proteins were measured, showing equal numbers of all ten proteins in the U1 spliceosomal snRNP. These data demonstrate that quantitative mass spectrometry has great potential for the determination of the stoichiometry of multiprotein complexes.