Isotope dilution gas chromatography-mass spectrometry (GC-MS) and electrothermal atomic absorption spectrometry (EAAS) are compared for platinum (Pt) determination in urine, plasma ultrafiltrate, and plasma samples from a patient undergoing cisplatin therapy. The isotope dilution GC-MS method is based on the use of lithium bis(trifluoroethyl)dithiocarbamate as a chelating agent and enriched 192Pt as an internal standard. Pt isotope ratios were measured using a Finnigan MAT 8230 organic mass spectrometer, and Pt concentrations were calculated from different sets of isotope ratios in the molecular ion of the Pt-chelate. In the EAAS method, Pt concentrations were determined using three different approaches. These were: (i) calibration curve based on aqueous standards containing Pt in 10% HCl, (ii) standard addition, and (iii) matrix digestion followed by standard addition. Good agreement was obtained for Pt concentrations determined by GC-MS and EAAS in urine samples while there were significant differences in Pt concentrations of ultrafiltrate and whole plasma samples by the two methods. Discussion of possible reasons for these differences emphasizes the need for future critical evaluation of these methods.