-macroglobulin was developed for quantitation of this large macromolecule in rat serum (or plasma) and tissue extracts. There was no cross reaction with α
2
-macroglobulin analogues which existed in rat serum. The sensitivity was 8.0 ng/ml and the mean recovery rate was 104.3%
with the intea and intra assay CV of 6.30% and 9.93% respectively. Finally
the levels of α
2
-macroglobulin in normal rat plasma and several tissue extracts were determined by this metnod.