Quartz sand columns and sand-filled microscope flow cells were used to study the transport characteristics of the clay colloid laponite, and a biofilm-forming bacterium, Pseudomonas aeruginosa SG81. Sep. expts. were performed with each particle to det. their individual transport characteristics in clean sand columns. In a 2nd set of expts., bacterial biofilms were formed prior to introduction of the clay colloids. In the independent transport expts., bacteria and laponite each conformed to known physicochem. principles. A NaCl concn. of 7 * 10-2M caused complete retention of the laponite within the sand columns. P. aeruginosa SG81 was generally less influenced by ionic strength effects; it showed relatively low mobility at all ionic strengths tested and some (albeit reduced) mobility when introduced to the columns in 1M NaCl, the highest concn. tested, but nevertheless showed reproducible trends. Under conditions favorable to laponite retention and biofilm stability (7 x 10-2M NaCl), laponite suspensions were able to remobilize a portion of the attached bacterial biomass. At low ionic strength, the profile of laponite elution was also altered in the presence of a P. aeruginosa biofilm. These observations suggest that while a redn. in ionic strength has a dominant influence on the mobilization of biol. and inorg. colloids, the presence of laponite and biomass can have a distinct effect on the mobility of both types of colloids. Since these events are likely to occur in subsurface environments, our results suggest that colloid-biofilm interactions will have implications for colloid-bound contaminant transport and the remobilization of pathogens.