The National Institute of Standards and Technology (NIST), in support of the National Oceanic and Atmospheric Administration s Marine Mammal Health and Stranding Response Program (NOAA/MMHSRP) conducts annual interlaboratory comparison exercises for the determination of chlorinated pesticides, polychlorinated biphenyl congeners, and trace elements in marine mammal tissues. These exercises provide one mechanism for laboratories to evaluate their measurement quality and comparability for these constituents in marine mammal tissues. In the 2003 exercise, 24 laboratories participated in determining the concentrations of selected polychlorinated biphenyl congeners (PCBs) and organochlorine pesticides in a homogenized blubber control material Marine Mammal Quality Assurance Exercise Homogenate VI (Homogenate VI) and Standard Reference Material (SRM) 1945 Organics in Whale Blubber. This report includes the results reported by the participating laboratories, combined consensus data results, and summary statistics for each analyte in the samples. The numerical indices used to assess laboratory performance are also discussed.