Field experiments in the Chernobyl exclusion zone have been carried out to determine the behaviour of the radioactive halogens 36Cl and 125I (as a surrogate for 129I) in the soil-plant system and along human food chains. The investigations on the migration in four types of soil of the two radionuclides showed that the vertical transport of chlorine was the most rapid: 9 months after the contamination the residual fractions of 36Cl in the 20 cm arable layer varied from 1% to 3% according to soil type. For iodine this percentage was less than 3% in any case. The soil to plant transfers were quantified for various agricultural crops (cereals, grass, root vegetables, leaves vegetables and fruit vegetables) and for several kinds of soils. The root uptake of 36Cl by plants was particularly important. As a result of this very high biological assimilation about 60% of the contamination was extracted from the soils by the plants after one vegetation period. For iodine the concentration ratios values are 2 or 3 order of magnitude lower than for chlorine.