The metal-binding capacities of some gel-forming polysaccharides and other substances have been investigated in vitro in an attempt to relate their metal-binding properties to the retention of dietary Pb and Cd in vivo. In equilibrium dialysis systems, aqueous solutions of alginic acid, pectin, agar, and carrageenan (1 g fiber/100 ml) all bound Pb and Cd to varying degrees. Alginic acid had the greatest binding capacity for Pb (50 micrograms Pb bound/mg fiber) and carrageenan for Cd (9.3 micrograms Cd bound/mg fiber). Addition of any one of these fibers, or indulin or glucuronic acid to the diet increased the tissue retention of one or both of the metals. Only cellulose supplementation reduced the retention of both Pb and Cd. Carrageenan decreased that of Pb and increased that of Cd. In another experiment alginic acid was shown to increase Pb retention in rats even when present at fairly low dietary concentrations (1 g/kg).