The eight-hour plasma absorption of cyanocobalamin Co 57 was performed in 186 patients with pernicious anemia, 40 with intestinal malabsorption, 147 with gastric resection, 51 with simple gastric achlorhydria or hypochlorhydria, and 141 controls. Patients with pernicious anemia and intestinal malabsorption could always be separated from control subjects by this method and, with the addition of hog intrinsic factor concentrate, usually from each other. The plamsa absorption test closely reflected the serum vitamin B12 concentration in untreated patients with pernicious anemia and intestinal malabsorption, but exceptions occurred in patients with gastric resection, simple hypochlorhydria, or achlorhydria. The plasma absorption test showed good correlation with the urinary excretion method. The plasma absorption of crystalline cyanocobalamin Co 57 is a valid reflection of vitamin B12 absorption in pernicious anemia and intestinal malabsorption.