Losses of immunoglobulins were measured in ten patients undergoing peritoneal dialysis. Mean immunoglobulin losses per 80 liter dialysis were as follows: IgG 17.9 gm, IgA 3.8 gm, and IgM 0.78 gm. Bleeding during dialysis generally accounted for less than 10% of these losses. lgA/lgG and lgM/lgG ratios in dialysate and serum were similar suggesting that the peritoneal membrane is permeable to higher molecular weight molecules. This was confirmed by the demonstration of peritoneal permeability to dextrans of greater than 150,000 molecular weight. In the absence of compensatory increases in synthesis, losses of immunoglobulins of the magnitude described in patients on chronic peritoneal dialysis programs could result in hypoglobulinemia and altered host resistance.