Dextran sodium sulfate was evaluated in vitro as a potential non-absorbable osmotic agent for peritoneal dialysis. It was compared to poly(sodium acrylate) which has been shown previously to be effective in rats, but probably toxic. Dextran sodium sulfate induced osmotic ultrafiltration rates as high as 20 ml/min in water but only 2 ml/min in solution containing non-polymer electrolytes presumably because of Gibbs-Donnan effects. Compared to acrylate the dextran polymer has less sodium per gram, lower osmotic activity of polymer sodium, and yields less ultrafiltration at given transmembrane osmolalities. A non-toxic polymer more like acrylate would seem more promising as an osmotic agent for peritoneal dialysis.