A high molecular weight form of angiotensinogen (HMrA) is present in low quantities in plasma from men, in moderate quantities in plasma from pregnant women, and in larger quantities in amniotic fluid. It was shown that immobilized IgG from anti-low molecular weight angiotensinogen (LMrA) sera effectively removed HMrA from all three sources. Immunoblots of sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) using anti-LMrA sera showed that HMrA from each of the three sources contained a subunit that was identical to LMrA with respect to molecular weight and heterogeneity. Thus, no difference in the HMrA from these three sources could be demonstrated. It is concluded that HMrA contains at least one subunit that is very similar if not identical to LMrA.