An abnormal microheterogeneous component of serum transferrin, with a higher isoelectric point than the normal main component, was studied by means of isoelectric focusing and direct immunofixation in 98 alcoholic patients, 22 patients with liver diseases and 100 controls. Its relation to acute and prolonged ethanol intake was studied in healthy volunteers. The abnormal transferrin component was found to be a sensitive indicator of prolonged, high alcohol ingestion, and was observed in 81% of patients with an admitted consumption of more than 60 g ethanol/day, and normalized after at least 10 days of abstinence. It occurred in 1% of the controls and in none of the cases with liver diseases without current alcohol abuse. There is evidence of a reduced sialic acid content in the abnormal transferrin. No similar change has been found in a number of other glycoproteins. This test may be a useful and sensitive tool for the detection of chronic alcohol consumption.