Although quantitative tests of some hepatic functions have been well established, the determination of serum cholinesterase activity continues to be commonly used in their stead. A critical comparison of the serum cholinesterase activity with these quantitative tests, however, is still lacking. Serum cholinesterase activity was therefore simultaneously compared with galactose elimination capacity (GEC), initial BSP-disappearance rate (BSP-ki), and serum albumin levels in 19 healthy control subjects and 46 patients with various chronic liver diseases. Serum cholinesterase activity was less discriminating between controls and patients than BSP-ki. It appears poorly suited, therefore, as a screening test for mild liver disease. Rank correlations between serum cholinesterase activity and GEC, BSP-ki, and serum albumin were statistically higher significant (r = 0.65, r = 0.74, and r = 0.80 respectively). On a statistical basis, serum cholinesterase activity may, therefore, be regarded as an index of the functional reserve of the liver. Evaluation of individual cases, however, revealed some clinically relevant discrepancies. It is concluded, therefore, that for accurate follow-up studies measurements of serum cholinesterase activity may be insufficient substitutes for the quantitative tests.