During a series of dialyses with the REDY absorbent dialysis system, samples for bacteriological examination were drawn from different parts of the system. The REDY cartridge, both unused and after dialysis, was submitted to bacteriological examination by the drawing of samples from its four layers. Bacteria grew from the different layers of the cartridge with decreasing frequency from the bottom layer upwards. There was bacterial growth both in samples taken from unused cartridges and in samples from cartridges at the conclusion of dialysis. An attempt to sterilize the bottom layer of the cartridge by irradiation had no apparent effect on the incidence of growth. There was no growth in the dialysate, while bacteria could be cultured from both sides of the membrane of the dialyzer as well as from the solution of concentrate. Bacteria are probably found in the urease layer of the unused cartridge and may multiply during dialysis, leak out into the dialysate, and be trapped in the dialyzer. It is concluded that bacteriological contamination of the system should be avoided by the use of a suitable and effective sterilization procedure.