Four bacteria, Bacillus cereus, B. subtilis, Escherichia coli, and Pseudomonas aeruginosa, were examined for the ability to remove Ag+, Cd2+, Cu2+, and La3+ from solution by batch equilibration methods. Cd and Cu sorption over the concentration range 0.001 to 1 mM was described by Freundlich isotherms. At 1 mM concentrations of both Cd2+ and Cu2+, P. aeruginosa and B. cereus were the most and least efficient at metal removal, respectively. Freundlich K constants indicated that E. coli was most efficient at Cd2+ removal and B. subtilis removed the most Cu2+. Removal of Ag+ from solution by bacteria was very efficient; an average of 89% of the total Ag+ was removed from the 1 mM solution, whereas only 12, 29, and 27% of the total Cd2+, Cu2+, and La3+, respectively, were sorbed from 1 mM solutions. Electron microscopy indicated that La3+ accumulated at the cell surface as needlelike, crystalline precipitates. Silver precipitated as discrete colloidal aggregates at the cell surface and occasionally in the cytoplasma. The results indicate that bacterial cells are capable of binding large quantities of different metals. (Copyright (c) 1989, American Society of Microbiology.)