The concentrations of 129I and the ratios of 129I/127I in normal human thyroids collected in Tianjin, China, and some seaweed samples from the Chinese coast were determined by neutron activation analysis. The mean 129I/127I 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 129I/127I 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 129I/127I 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 129I level in China is within the global fallout background level.