Colloid transport through porous media can be influenced by the presence of biofilms. Sterile and non-sterile sand columns were studied using Laponite RD as model colloid and a highly mucoid strain of Pseudomonas aeruginosa as model biofilm former. Laponite RD was marked specifically by fluorescent complexes with Chodamine 6G. Breakthrough curves (BTCs) were used as parameters for detn. of colloid transport characteristics. In the sterile columns, the colloid was mobile (collision efficiencies from 0.05 to 0.08) both after the presence of Na+ and Ca2+ followed by deionized water influent. In the biofilm-grown column, the same treatment did not result in colloid retention in the case of Na+ exposure, but in altered or enhanced colloid transport. In the case of Ca2+ exposure, colloid retention increased with biofilm age. After 3 wk, almost complete retention was obsd. Similar observations were made in columns packed with material from slow sand filtration units. These data reveal the complex interactions between biofilms, cations and colloid transport. Changes in the electrolyte compn. of water percolating the subsurface can frequently occur and will result in different colloid transport characteristics with regard to the dominating species of ions and the relative abundance of microbial biofilms. This has to be considered when modeling colloid transport through the subsurface.