In the present study we have used an enzyme-linked immunosorbent assay to measure the immunoreactivity towards acetaldehyde-modified proteins in plasma from alcoholics, patients with non-alcoholic liver disease and control social drinkers. All three groups showed a response to modified proteins when total immunoglobulin binding was measured. However, when the assay was modified such that class-specific immunoglobulin binding was measured, it was found that the alcoholic response had a significantly greater immunoglobulin A component than patients with non-alcoholic liver disease or controls who were social drinkers. Thus measurement of IgA binding to acetaldehyde-modified proteins may be a marker for alcohol abuse.