The decline in Cs radioisotope levels has been studied in tissues from lambs grazing lowland pasture. The lambs were slaughtered 18 and 115 d after contamination with Chernobyl fallout. During this time the Cs activity decreased to 13% of the initial amount in animals that had continued to graze contaminated pasture and to 3.5% in animals consuming uncontaminated feed. The 137Cs concentration in grass from the field grazed by the lambs decreased with a half-time of 22 d over the period 11-100 d after contamination. The amounts of Cs radionuclides removed from the pasture by the grazing animals amounted to only 0.01% of the total, the rest remaining in the soil, with over 40% in the upper 10 mm. Small amounts of 110mAg, found in grass, declined with a half-time of 8.9 d, and the radionuclide was found to accumulate in liver tissue.