Bile salts have been shown to act on lipoprotein-X (LP-X) in vitro to induce a false-negative electrophoretic test. The aim of the present study was to investigate the relationship between serum LP-X and serum bile acids in patients with cholestasis. The in vitro concentration of bile salts required to induce a negative or reduced concentration of LP-X was also studied. There was no relationship, either positive or negative, between serum LP-X and bile acids in 34 patients with cholestasis. Serum was incubated with various saline solutions of taurocholic, lithocholic, deoxycholic and glycocholic acids. The concentration of LP-X decreased only after the final concentrations of bile salts were over 2,000 µmol/l. This is more than five times the concentration of serum bile salts usually found in patients with cholestasis. It is concluded that the negative LP-X test in some patients with cholestasis must be explained by some other mechanisms than bile salts