A brief summary is given of studies of the flocculation of silica, algae and bacteria by means of low concentrations of synthetic polymers. The effects of polymer molecular weight and structure have been emphasized, using the silica, and pure cultures of green algae and E. coli under a variety of conditions of pH and colloid concentration. Filtration rate, light transmission and electrophoretic mobility have been employed to measure degree of flocculation. Non-ionic and anionic polymers did not produce flocculation, but cationic polyethyleneimines were effective at concentrations of 0.1 to 100 ppm, the optimum being dependent on pH, colloid concentration and polymer molecular weights above about 2000. Two types of polyethyleneimine polymer tagged with radioactive C14 were prepared which flocculated the three kinds of colloids in the same way as untagged polymers. Poor flocculation efficiency was not to be associated with low adsorption, but to specific polymer-surface interactions which reduced the efficiency of the adsorbed polymer. The results have been shown to conform with current theories of flocculation of other colloids. They should assist in attaining better water quality control by the use of the synthetic polymers in numerous practical operations. (Author)