Major advances in dialysis therapy have occurred over the last decade, yet various abnormalities persist in end-stage renal disease (ESRD) patients. The etiology of these residual defects remains largely unknown. We are currently testing the hypothesis that some of these abnormalities are due to retention of small molecular weight, protein bound toxins, which are poorly dialyzable. We sought an alternative to blood as a source of bound toxins. Spent peritoneal dialysate (PD) was tested as a source. With use of a series of filtration devices, PD albumin content was increased about 35-fold. Evidence of bound ligands was shown by two methods. Salicylate binding by patients' sera and concentrated PD (n = 8) were markedly reduced, unbound salicylate being 14.9 +/- 5.1% (SD) and 15.8 +/- 4.9% at albumin concentrations of 3.30 +/- 1.04 and 3.23 +/- 0.84 g/dl. Serum from eight normal subjects, diluted to 2.95 g/dl albumin, had 7.4 +/- 1.1% unbound salicylate. HPLC analysis of deproteinized concentrated dialysate was compared to ultrafiltrates of the same fluid. Numerous bound peaks were seen, particularly in the late eluting peaks. Spent PD is a rich source of protein bound ligands for further study.