Nonfragmenting field ionization mass spectrometry is being applied in conjunction with multilabeled organic compounds as a means of assaying chemotherapeutic agents and metabolites in biological samples by isotope dilution analysis. Multilabeled metabolites are also being used as tracers of physiological process, e.g., hexalabeled thymidine is being used to measure the rate of DNA synthesis. The same mass spectrometric technique is also being used for multicomponent analysis of metabolites in biological fluid as a means of finding biochemical patterns significantly correlated with cancer.