Abstract: Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS) was used for the study of complexes formed by yeast seryl-tRNA synthetase (SerRS) and tyrosyl-tRNA synthetase (TyrRS) with tRNASer and tRNATyr. Cognate and noncognate complexes were easily distinguished due to a large mass difference between the two tRNAs. Both homodimeric synthetases gave MS spectra indicating intact desorption of dimers. The spectra of synthetase-cognate tRNA mixtures showed peaks of free components and peaks assigned to complexes. Noncognate complexes were also detected. In competition experiments, where both tRNA species were mixed with each enzyme only cognate alpha2.tRNA complexes were observed. Only cognate alpha2.tRNA2 complexes were detected with each enzyme. These results demonstrate that MALDI-MS can be used successfully for accurate mass and, thus, stoichiometry determination of specific high molecular weight noncovalent protein-nucleic acid complexes.