The concentrations of I-129 and the ratios of I-129/I-127 in normal human thyroids collected in Tianjin, China, and some seaweed samples from the Chinese coast were determined by neutron activation analysis. The mean I-129/I-127 ratio in these thyroids was found to be 1.13 x 10(-9), which is two orders of magnitude higher than the level of the pre-nuclear era, but one order of magnitude lower than the level in Europe in the post-nuclear era. There is no significant difference between the ratio of I-129/I-127 in the thyroids for the post-nuclear era from China and other areas, which are considered not to have been directly exposed to 129I emission from a nuclear source, such as Chile, Taiwan and Tokyo. The mean I-129/I-127 ratio in seaweed from the Chinese coast is 2.35 x 10(-10), approximately two orders of magnitude higher than in seaweed collected in the pre-nuclear age, and similar to that from locations without direct exposure to the emission from nuclear installations, influenced only by global fallout. This indicates that the I-129 level in China is within the global fallout background level. (C) 2000 Elsevier Science B.V. All rights reserved.