Sera from a group of 79 patients with pernicious anemia were studied for the presence of antibody to intrinsic factor. Two general types of antibody activity were found, and it was possible to distinguish three groups of pernicious anemia sera on the basis of their content of these types. Type I antibody blocks the binding of radioactive vitamin B(12) to intrinsic factor when added to intrinsic factor before the B(12); it is not detected on intrinsic factor when added after B(12). This antibody blocks intrinsic factor-mediated B(12) absorption in vivo when mixed in the sequence intrinsic factor + antibody I + B(12), but not when mixed in the sequence intrinsic factor + B(12) + antibody I. Type II antibody reacts with intrinsic factor when B(12) is attached. This antibody prevents the absorption of B(12) from intrinsic factor in pernicious anemia patients when mixed in the sequence intrinsic factor + B(12) + antibody II, and is thereby distinguished from antibody I.