[en] To coordinate the isotope measurements in India and to make these measurements traceable to the national Primary Standards, a national intercomparison of activity measurements with the radioisotope iodine-131 was organized. This is the first intercomparison and iodine-131 was selected because it is extensively used in nuclear medicine. Other radioisotopes of interest will be taken up in future. Standardized solutions of iodine-131 were sent to the participants without disclosing the true activity of sample. They were asked to measure on their detection system and report the activity of the sample alongwith the details of the measuring system. This report describes briefly the organization of the intercomparison, preparation of samples of iodine-131, primary standardization and international intercomparison. The results of measurement as well as the details of measuring systems used by the participants are included. Finally, based on the experience of this intercomparison some recommendations are given for the participants to enable them to make measurements with better accuracy and reliability. (author)