—With their additional case, King and Toskes raise several germane speculations and questions. Clearly, we need to understand better the process that is measured by testing protein-bound cobalamin absorption, whether by using cobalamin incorporated into eggs and other foods as developed by Doscherholmen et al,1,2 or cobalamin bound to chicken serum binders as used by King and Toskes. We also need to establish the mechanism, importance, and natural history of such malabsorption to know how to respond to abnormal results.Two clinical issues seem paramount. One is how often this subtle abnormality of absorption evolves into overt pernicious anemia accompanied by an abnormal Schilling test result. So far, King and Toskes and I have each been dealing with highly selected cases. Nevertheless, these observations suggest that systematic prospective study may well disclose such a progression to be common, perhaps even common enough to call for prophylactic treatment.