Complex formation between purified Gc and G-actin caused increased rocket height on immunoelectrophoresis with monospecific Gc antiserum, and artifactually high calculated Gc levels. The increase in rocket height varied in log: linear fashion with the amount of G-actin present, up to a plateau attained at equimolarity. The raw Gc values could therefore be corrected to within +/- 10% of known levels by addition of excess G-actin and use of standard plots obtained with Gc after saturation with G-actin. This also allowed quantitation of the percentage of Gc complexed with G-actin. In subsequent studies of whole human sera, comparison of normal controls with pregnant subjects and patients with liver disease showed evidence of differences both in absolute quantities of Gc and the relative proportion circulating as complex with G-actin. This appeared to be due to increased release of cellular actin into the extracellular space. These results show that rocket immunoelectrophoresis can be modified to provide accurate Gc levels, and also information concerning different molecular forms of this protein.