Gene therapy is expected to lead to new and useful methods to treat diseases. The development of assays to quantitate gene-therapy-derived proteins circulating in blood will be essential to investigate the effects and side effects of the introduced proteins. The purpose of this study is to evaluate whether a protein circulating at trace concentrations in blood can be measured by tagging a peptide corresponding to glucagon residues 19-29 onto its C-terminal end. We constructed plasmids encoding chimeric proteins and transferred them into rats by hydrodynamics-based delivery. When plasmids encoding human IL8-glucagon 19-29 chimeric protein were injected into rats to evaluate the accuracy of this method, there was a high correlation between chimeric proteins measured by an enzyme-linked immunosorbent assay for human IL8 and one by a radioimmunoassay for glucagon. Furthermore, when plasmids coding rat IFN gamma receptor IgG-Fc glucagon 19-29 chimeric protein were injected to evaluate the time course of chimeric proteins in blood plasma, we could calculate the concentrations in blood from 10 microl plasma samples using glucagon 19-29 tag as follows: 2815+/-2318 ng/ml after 4 hours (mean+/-s.D.), 6061+/-2789 ng/ml after 8 hours, 5752+/-2270 ng/ml after 12 hours, 2870+/-1062 ng/ml after one day, 1440+/-334 ng/ml after three days, 1120+/-433 ng/ml after seven days, and 281+/-162 ng/ml after 16 days. Blood sugar levels which might have been increased by glucagon did not increase even at peak chime- ric protein concentrations. These results demonstrate a useful and convenient method to assay gene therapy products circulating in blood using a glucagon 19-29 tagging vector.