The persistence of high levels of 134Cs and 137Cs in vegetation of certain upland areas of the United Kingdom has prompted investigations into possible methods of reducing levels in sheep tissues. The introduction of the clay mineral bentonite into the diet of ruminants has been shown to reduce the uptake of caesium from the gut. The effects of treating pasture with bentonite at two rates (single or repeated application of 80 g m-2) on the intake and transfer of 137Cs to sheep tissues was investigated. Whilst there were reductions in the radiocaesium activity concentrations of tissues of sheep grazing pasture treated at both rates, transfer coefficients were only affected for sheep on the repeated treatment. However, ewes grazing the repeatedly treated pasture experienced a loss in body weight (18%) associated with a decrease in herbage intake (395). Considerable variation in the herbage intake by individual ewes and lambs, both between and with treatments, showed the need for direct intake measurements in order to determine relevant transfer coefficients in field experiments. Transfer coefficients for 137Cs of 0.33 d kg-1 and 1.61 d kg-1 were obtained for muscle of control ewes and lambs respectively.