A qualitative change of the microheterogeneity of serum transferrin, demonstrated by isoelectric focusing, has previously been found to occur frequently and with high specificity in alcoholic patients during current abuse, and has been proposed as a new marker of alcoholism. Certain indirect evidence has supported the assumption that the basis for the altered transferrin heterogeneity would be a reduced sialic acid content. In this investigation serum transferrin from healthy controls and alcoholic patients was purified by affinity chromatography on antitransferrin Sepharose 4B. The sialic acid content in transferrin was thereafter determined directly. Transferrin from alcoholic patients showed a 22% lower sialic acid concentration than control transferrin, which was highly significant. These data together with results from experiments with neuraminidase and galactose-binding lectin provide evidence that in alcoholism at least two sialic acid residues are missing in a significant fraction of serum transferrin. This observation may indicate that sialic acid residues are missing in a significant fraction of serum transferrin. This observation may indicate that sialic acid metabolism is one important target of the biological action of ethanol.