During last few years were reports concerning a new endogenous digoxin -like immunologic factor (DLIF) in patients not receiving digitalis. DLIF was stated in pregnant women's blood umbilical and neonatal blood as well as in patients with renal failure or hepatopathy. This phenomenon could be related to hormones changes (pregnancy) as well as alterations of cholesterol, triglycerides, free fatty acids, albumins or total protein level (nephropathy, hepatopathy). DLIF overstated determination of serum digoxin concentration, which in the case of exceptional narrow digoxin therapeutic spectrum as well as its concentration-dependent toxicity became a significant clinical problem. Effect extent of DLIF on serum digoxin level could be also related to applied analytic technics. The aim of the study was to compare two routinely applying analytic methods: polarized immunofluorescence (FPIA-TDx Abbott) with radioimmunological assay (RIA). The study was performed in patients with renal failure to estimate DLIF effect on real serum digoxin concentration as well as on extent and DLIF elimination velocity during dialysis. DLIF occurrence in patients with renal failure not receiving digitalis was experimentally stated using both RIA and FPIA methods. However, RIA revealed DLIF in all cases: before and after dialysis as well as in not dialyzed patients with a concentration above 0.3 ng/ml, when FPIA values were respectively: 0.087; 0.043; 0.078 ng/ml, which was less than 10% of digoxine therapeutic range (0.9-2.0 ng/ml). DLIF lowered in a course of dialysis to FPIA advantage, which was proved by FPIA/RIA ratio decrease nearly of a half of its predialysis value. Pre- and postdialysis values of FPIA/RIA ratio were 0,204 and 0,134 respectively.(ABSTRACT TRUNCATED AT 250 WORDS)