[en] The International Laboratory of Marine Radioactivity (ILMR) at Monaco, following a request given to it by CRESP (Coordinated Research and Environmental Surveillance Programme for Sea Disposal, OECD/NEA) organized during 1986-1987 an intercomparison exercise for determining the concentrations of natural and artificial radionuclides in a sample of deep sea sediment. Including the nine CRESP participants, samples were sent to fifty-one laboratories in twenty-eight countries. It was anticipated that the concentrations of the artificial radionuclides in this sediment would be at or below analytical detection limits. Participating laboratories were requested to determine as many radionuclides as possible from the following list: ⁶⁰Co, ⁹⁰Sr, ¹³⁷Cs, transuranium elements, natural radionuclides such as ⁴⁰K, uranium isotopes, thorium isotopes, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po. In the information sheet provided it was stated that the concentrations of artificial radionuclides were very low and could be close to detection limits. The analysts were requested to make, whenever possible, at least three determinations for each radionuclide reported. They were also asked to report net rate activity in the sample, as either a negative or a positive value. An Atlantic marine sediment was collected in Autumn 1985, by box-corer from the area 46 deg. to 48 deg.30' North and 18 deg. to 22 deg. East. The water depth in that area is more than 4000m. Activities of the natural radionuclides in the selected samples did not deviate by more than 8%. Thirty-four laboratories from twenty countries have reported data on the following radionuclides ⁴⁰K, ⁶⁰Co, ⁹⁰Sr, ²¹⁰Pb, ²¹⁰Po, ¹³⁷Cs, ²²⁶Ra, ²²⁸Ra, ²²⁸Th, ²³⁰Th, ²³²Th, ²³⁴U, ²³⁵U, ²³⁸U, ²³⁸Pu, ^239+240Pu and ²⁴¹Am. 6 refs, 12 tabs