Crossed immunoelectrophoretic patterns were obtained on three patients with a congenital deficiency of anti-thrombin III (AT-III), and on normal single donor and “pooled” plasma controls. In the presence of heparin (incorporated into the agarose gel matrix), the AT-III of normal human plasma was separated into four components: two major, faster-moving components, and two minor, slower-moving components. The three patients with congenital deficiency of AT-III (levels approximately 50% of normal) appeared to possess only one of the faster- moving components, and one of the slower-moving components.