Patients may fail to absorb cobalamin (vitamin B12) bound to food even when they have adequate intrinsic factor to absorb free cobalamin normally. We studied cobalamin transfer from egg yolk cobalamin-binding protein to human saliva and gastric juice as a model of this important first step in cobalamin assimilation. The cobalamin-binding protein of egg yolk eluted with human R binder on Sephadex gel chromatography and bound cobalamin with a comparable affinity, but it did not cross-react with R binder immunologically. Transfer of cobalamin from egg yolk to saliva or gastric juice R binder did not occur at neutral pH. Slight transfer (8%-12% of the 57Co-cobalamin bound to egg yolk) occurred when the saliva was acidified to pH 1.5. This minor transfer by acid was not inhibited by pepstatin A, a pepsin inhibitor. Acidification caused variable transfer to gastric juice R binder (12%-40%) that appeared to be partially due to residual gastric pepsin activity. Adding 1200 U of pepsin per milliliter enhanced cobalamin transfer to saliva or gastric juice R binders (39%-58% transfer). At no time was cobalamin transferred directly to intrinsic factor; R binder-deficient gastric juice failed to accept cobalamin from egg yolk. The transfer of cobalamin from egg yolk to human R binder requires both an acid pH and pepsin activity. While as little as 30 U of pepsin added per milliliter of saliva promoted transfer of cobalamin, the requirement for an acid pH was very strict. Virtually no transfer occurred when pH exceeded 2.0, regardless of the amount of pepsin present. Acid provided an optimal pH for pepsin activity and, to a lesser extent, affected transfer by a mechanism unrelated to pepsin. Our data suggest that compromised pepsin secretion and, probably even more importantly, compromised acid secretion interfere with transfer of food cobalamin to R binder.