The in vitro binding characteristics of radioactive 137Cs to two forms of Prussian blue [colloidally (soluble) K3Fe[Fe(CN)6] and insoluble Fe4[Fe(CN)6]3] and to activated charcoal and sodium polystyrene sulfonate (Resonium-A) were investigated by constructing Langmuir isotherms at pH = 1.0, 6.5 and 7.5 at 37 degrees C. At the three pHs investigated, 137Cs binding to activated charcoal and sodium polystyrene sulfonate was negligible. Binding of 137Cs to insoluble Prussian blue exceeded that for the soluble form and was pH dependent for both formulations. Maximum binding capacities were 87 mg/g (pH = 1.0), 194 mg/g (pH = 6.5) and 238 mg/g (pH = 7.5) for the insoluble form and 48 (pH = 1.0), 73 (pH = 6.5) and 78 (pH = 7.5) for the soluble form. As activated charcoal did not bind 137Cs, charcoal hemoperfusion is of no value. This has been confirmed by an in vitro experiment, using a Gambro Adsorbs 300 C cartridge.