1. Field investigations into the transfer of 137Cs from pasture to sheep tissues have been made at two contrasting sites in west Cumbria, close to the nuclear fuel reprocessing plant at Sellafield. These were a saltmarsh bordering on the Esk estuary in 1982 and inland pastures close to the perimeter of the work in 1984. 137Cs concentrations of samples vegetation from the saltmarsh were generally two orders of magnitude greater than from the inland pasture due to its inundation with 137Cs associated with silt. The relatively high 137Cs content of soil/silt compared to that of vegetation at each site meant that soil contamination of vegetation sometimes accounted for a substantial proportion of its 127Cs activity (up to 99% on the saltmarsh and 67% on the inland pastures). Considerable seasonal changes occurred in the extent of 137Cs contamination on the inland pastures, with late winter and early spring levels being up to 20-fold higher than those of the summer. 2. 137Cs concentrations in tissues of lambs from the saltmarsh were consistently higher than for ewes; this was not true for sheep from the inland pastures. 137Cs concentrations in kidney were found to be higher than in all other tissues, both in the study of flocks and in one of the controls. 3. Transfer coefficients (calculated by dividing the 137Cs concentration of fresh tissue by the daily intake of 137Cs) were significantly (P<0.05) higher in saltmarsh sheep than in the inland pasture sheep; the difference in transfer coefficients was increased further if 137Cs activity due to soil/silt contamination was removed from the estimates of daily intake, leaving 137Cs associated with the vegetation only to contribute to the transfer coefficients. Further studies are required too determine tha availability of 137Cs associated with soil/silt particles which are ingested by animals.