Isotopically labeled ubiquitin is useful in determining total cellular concentrations of ubiquitin, or determining the ratio of free ubiquitin to poly-ubiquitin chains using the protein standard absolute quantification (PSAQ) or related methods. Highly purified 13C/15N labeled ubiquitin processed for the quantitative removal of glycine and buffer salts which can interfere with chemical and in vitro reactions. Ubiquitin is a 76 amino acid, highly conserved nuclear and cytoplasmic protein.