Values of various blood plasma tests have been determined in heavy drinkers admitted to an Alcoholism Unit and in a reference group of occasional social drinkers. These tests were total protein, albumin, bilirubin, alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), aspartate amino transferase (AST) and amylase. The aim of this study was to assess the reliability and validity of GGT and AST as indices of alcohol intake in heavy drinkers entering an Alcoholism Unit. The data indicate that in the patients group intra-personal variance and analytical variances are relatively small compared with the between-person variance. The data also indicate, however, that in the patients group GGT and AST are not valid group indices of alcohol intake since the between-person variation is large and the correlation with alcohol intake is weak due to large differences in the dose-response relationship between individuals. AST and GGT are poor indicators of alcohol intake in admissions to an Alcoholism Unit.