An interlaboratory comparison exercise for I-129 has been organized and conducted. A total of seven laboratories participated in the exercise to either a full or limited extent. In the comparison, a suite of 11 samples was used. This suite of standards contained both synthetic ''standard type'' materials (i.e., AgI) and environmental materials. The isotopic I-129/I-127 ratio of the samples varied from 10(-8) to 10(-14). Preliminary results of the comparison are presented.