In vitro experiments were performed to analyze problems with the determination of erythropoietin in dialysate. Human recombinant erythropoietin (EPO; 4000 U/L) was added to several fluids, to glass or polystyrene tubes with or without addition of bovine serum albumin (BSA) and to dialysate bags. The recovery in peritoneal effluent was 4120 +/- 203 U/L (mean +/- SEM). The recovery in the other fluids was less than 50% but in glass tubes it increased to 96% after addition of BSA. Binding was also found to the dialysate bag, therefore reducing the amount available for absorption. We recommend that EPO samples from the dialysate are drawn within BSA coated glass tubes.