[en] Within the framework of the International Chernobyl Project in order to assess the reliability of results obtained by different USSR laboratories it was decided that selected USSR laboratories involved in analysis of environmental contamination in the USSR due to the accident would be invited to participate in intercalibration exercises. These exercises were performed with two types of materials: 1) materials already available in the IAEA's Analytical Quality Control Services; 2) the vegetation material (clover) which was being used in a worldwide IAEA intercomparison programme in 1990. In May 1990 the Agency's Laboratories at Seibersdorf prepared a package of material for all participating laboratories, consisting of the following samples: (1) two bottles of milk powder (low and high level of activity), simulated air filters, one bottle of soil; (2) one bottle of the vegetation material (clover). This package was accompanied by detailed information sheets on its use and standardized data reporting format. The milk powder, soil and clover samples were distributed to additional laboratories in October 1990