We have studied the effect of indican on six commercial procedures for the measurement of total bilirubin in serum. Total bilirubin measured by the Bilirubin A-Gent (Abbott) 2,4-dichlorophenyl diazonium procedure increased by 50 mg/L for each 1 mmol/L of added indican. Similarly, total bilirubin measured by the Bilirubin C-System (Boehringer Mannheim) 2,5-dichlorophenyl diazonium procedure increased by 33 mg/L per mmol/L of indican. Indican also interfered with the Micro Bilirubin Reagent Set (Harleco) Malloy-Evelyn procedure, but to a much lesser extent. The Jendrassik Bilirubin Reagent System (American Monitor) and a modified Jendrassik-Grof procedure (Hoffmann-LaRoche) adapted to the Cobas Bio analyzer were unaffected by the presence of indican. The amount of interference with the 2,5-dichlorophenyl diazonium procedure increased significantly with color development time and was twice the initial amount after 30 min. Concentrations of indican as high as 0.38 mmol/L have been found in sera of patients with renal failure, which would increase total bilirubin values measured by the first two procedures above by 19 and 12 mg/L, respectively. Users of these procedures should therefore be suspicious of unexpectedly high bilirubin values obtained with sera from patients with chronic renal disease.