Serum levels of group-specific component (Gc) protein are useful in evaluating the likelihood of survival in patients with acute liver failure (ALF) who may be candidates for liver transplant surgery. Most methods for quantifying Gc protein concentration are either isotopic, manual, technically demanding, and/or time consuming to perform, and thus are not well suited for routine clinical use in a hospital setting. We modified and evaluated a recently described nonisotopic, fully automated, immunonephelometric method for quantifying serum Gc protein concentration and compared it to our previous immunoblotting method. In addition, we evaluated the effect of G-actin on the immunonephelometric measurement of Gc protein. Serum samples from 20 patients with ALF and from 20 age-and sexmatched clinic patients without liver disease were quantified by both immunoblotting and immunonephelometry. We assessed the intra-assay precision, correlation, and diagnostic accuracy of these methods in discriminating between individuals with no preexisting liver disease and those with ALF. Actin in 1.3-to 4-fold excess of Gc protein levels demonstrated minimal to no interference in the quantification of Gc protein by immunonephelometry. Immunonephelometry was more precise than immunoblotting. Gc protein values by immunonephelometry were similar to those obtained by immunoblotting, and the diagnostic accuracy of Gc protein concentration by immunonephelometry was similar to that observed by immunoblotting. Immunonephelometry provides a nonisotopic, fully automated, rapid, precise, accurate, and cost-effective method for quantifying serum levels of total Gc protein that is well suited for routine use in a hospital-based clinical laboratory.