Serum aspartate aminotransferase (AST) concentrations are commonly determined to detect hepatocellular damage. However, discrepancies between serum AST values and histological signs of active liver damage sometimes occur in patients with cirrhosis. The enzyme AST requires pyridoxal-5-phosphate (PLP) (active vitamin B6) as a co-enzyme to express its activity. Since approximately 90% of patients with severe cirrhosis are vitamin B6-deficient, it has been suggested that vitamin B6 supplements given to these patients might cause an elevation of falsely low serum AST concentrations. Treatment of 8 vitamin B6-deficient cirrhotic patients with pyridoxine hydrochloride (50 mg intravenously twice daily for 1 week) increased their serum AST concentrations from 121 +/- 18 (mean +/- SEM) to 136 +/- 26 lU/l, while treatment of a second group of 9 patients with the active co-enzyme PLP increased AST concentrations from 118 +/- 17 to 146 +/- 20 lU/l. Neither of these increases was statistically significant. Plasma PLP increased from 2,4 +/- 0,7 to 18,5 +/- 7,6 ng/ml after pyridoxine, and from 3,3 +/- 0,7 to 27,0 +/- 6,2 ng/ml after PLP supplementation. It is concluded that B6 deficiency is unlikely to be an important determinant of serum AST concentrations in patients with chronic liver disease.