[en] The document describes the outcome of the CSN/CIEMAT-05 interlaboratory test comparison among environmental radioactivity laboratories. The exercise was organised according to the ISO-43 and the IUPAC ^International harmonised protocol for the proficiency testing of analytical chemistry laboratories^. The exercise has been designed to evaluate the capability of national laboratories to determine environmental levels of radionuclides in vegetable ash samples. The sample has been prepared by the Environmental Radiation Laboratory, from the University of Barcelona, and it contains the following radionuclides: Sr-90, Pu-238, Am-241, Th-230, Pb-210, U-238, Ra-226, K-40, Ra-228, TI-208, Cs- 137 and Co-60. Reference values have been established TROUGH the kind collaboration of three international laboratories of recognized experience: IAEA MEL and IRSN-Orsay. The results of the exercise were computed for 35 participating laboratories and their analytical performance was assessed using the z-score approach. Robust statistics of the participant's results was applied to obtain the median and standard deviation, to achieve a more complete and objetiva study of the laboratories' performance. Some difficulties encountered to dissolve the test sample caused a lower response of analyses involving radiochemical separation, thus some laboratories couldn't apply their routine methods and no conclusions on PU-238, Am-241 and Th-230 performances have been obtained. The exercise has revealed an homogeneous behaviour of laboratories, being statistical parameters from the results close to the reference values. The study has shown that participant laboratories perform radioactive determinations in vegetable ash samples with satisfactory quality levels. (Author) 6 refs