The metal-binding affinity of the anionic poly-gamma-d-glutamyl capsule of Bacillus licheniformis was investigated by using Na, Mg, Al, Ca, Cr, Mn, Fe, Ni, and Cu. Purified capsule was suspended in various concentrations of the chloride salts of the various metals, and after dialysis the bound metals were analyzed either by graphite furnace atomic absorption spectroscopy or by inductively coupled plasma-mass spectrometry. Exposure of purified capsule to excess concentrations of Na revealed it to contain 8.2 mumol of anionic sites per mg on the basis of Na binding. This was confirmed by titration of the capsule with HCl and NaOH. Other metal ions were then added in ionic concentrations equivalent to 25, 50, 75, 100, 200, and 400% of the available anionic sites. The binding characteristics varied with the metal being investigated. Addition of Cu, Al, Cr, or Fe induced flocculation. These metal ions showed the greatest affinity for B. licheniformis capsule in competitive-binding experiments. Flocculation was not seen with the addition of other metal ions. With the exception of Ni and Fe all capsule-metal-binding sites readily saturated. Ni had low affinity for the polymer, and its binding was increased at high metal concentrations. Fe binding resulted in the development of rust-colored ferrihydrite which itself could bind additional metal. Metal-binding characteristics of B. licheniformis capsule appear to be influenced by the chemical and physical properties of both the capsule and the metal ions.