Within the last five years the International Laboratory of Marine Radioactivity at Monaco has organized seven intercomparison exercises worldwide on natural materials of marine origin comprising sea water (SW-N-2), sediment (SD-N-1/1, SD-N-1/2, SD-N-2), seaweed (AG-B-1) and fish flesh (MA-B-3/1, MA-B-3/2). Results on man-made (54Mn,60Co,65Zn,90Sr,99Tc,134Cs,137Cs,238Pu,239,240Pu,241Am) and natural radionuclides were collected and evaluated. Reference values were established for a number of them. In the paper are discussed various aspects of the intercomparison exercises.