Chromatography on Bio-Gel P-2 and high-performance liquid chromatography on RP-18 columns monitored with UV, fluorescence and electrochemical detectors have been used to evaluate the efficiency of granular, uncoated, active charcoal to remove from the ultrafiltrates of uraemic patients those organic substances accumulated in the blood that are not easily removed by dialysis. Chromatography on Bio-Gel P-2 and high-performance liquid chromatography on RP-18 columns carried out isocratically and monitored with an electrochemical detector seem very useful for clinical investigation as they increase the information obtained from routine haematochemical analyses such as blood urea nitrogen, creatinine, uric acid and electrolytes (calcium, phosphorus, sodium and potassium).