The reference values for gamma-glutamyltransferase (GT), aspartate aminotransferase (ASAT) and alkaline phosphatase (AFOS) activities in serum have been produced on the basis of measurements done in the Mini-Finland Health Survey. A representative sample of all Finns aged 30 years or over comprised 8000 persons, of whom 99.2% participated in the actual health survey. Every effort was made to obtain reference values for the healthy ambulatory population. Three separate health-derived selection criteria were used to obtain such reference values for the above-mentioned enzymes: those based on the available literature, with minor modifications, the recommendations published by the Committee on Reference Values of the Scandinavian Society for Clinical Chemistry and Clinical Physiology, criteria that were obtained after subgroup comparisons of the obtained data, where all the factors affecting the enzyme levels were identified. The recommendations of the Expert Panel on Theory of Reference Values (1987) were strictly adhered to in the statistical analyses. The distribution of the serum activity of GT was very skewed. The overall intervals for men and women were 5.1-1460 and 4.7-748 U/l, respectively. The frequency distributions could be transformed to the normal ones logarithmically. The 95% inner reference intervals for GT in the three groups were 7-76, 7-65, and 8-57 U/l for men and 6-35, 6-30 and 6-32 U/l for women, respectively. For ASAT the full intervals of the enzyme levels in serum were 2.6-770 U/l for men and 8.3-172 U/l for women. After logarithmic transformation the respective reference intervals in the three selection groups were 14-42, 14-40 and 13-39 U/l for men and 13-33, 12-31 and 13-33 U/l for women. The full intervals of AFOS were 47.5-2755 and 5.4-816 U/l for men and women, respectively; after the logarithmic transformation the reference intervals of the three selection groups were 98-267, 97-254 and 97-264 U/l for men and 77-265, 75-231 and 75-250 U/l for women, respectively.