Saccharomyces cerevisiae, Escherichia coli, and Pseudomonas putida homogenates were treated with polyethyleneimine (PEI) of various molecular weights and concentrations, and the removal of cellular contaminants noted. The optimum doses were determined and the effects of ionic strength observed. Although there was little variation between the abilities of the various PEIs to remove cellular contaminants, the possibility of floc redissolution at higher polymer concentrations occurred more with increased molecular weight of the polymer. Increasing the ionic strength decreased the occurrence of floc redissolution. Both E. coli and P. putida required a higher concentration of PEI than S. cerevisiae to achieve significant removal of cellular contaminants. Loss of soluble protein by absorption to flocs with E. coli was influenced strongly by ionic strength.