MRMPath is an online tool that predicts suitable peptides for multiple-reaction monitoring (MRM) mass spectrometry assays based on protein sequence. Users first enter the accession number or sequence of their protein of interest. Alternatively, users can choose a particular biological pathway and species, and MRMPath generates a list of all the proteins involved in the chosen pathway using information stored in the Kyoto Encyclopedia of Genes and Genomes database (KEGG). Registration not required.