Summary A micro-method for the detection of both intrinsic factor autoantibody and intrinsic factor which combines the techniques of immunodiffusion and immunoelectrophoresis with radioautography has been described. A radioactive precipitin are demonstrable by radioautography formed when a mixture containing a complex of human intrinsic factor (IF) and Co60B12 (IF-Co60B12) and rabbit anti-human IgG (RAHgG) was reacted against the sera from 8 of 32 patients with pernicious anemia. Radioimmunodiffusion and radioimmunoelectrophoresis experiments revealed that antibody was present in IgG of both type K and type L; antibody was not found in IgA or IgM. All antibody-containing sera blocked Co60B12 uptake by intrinsic factor. A nonradioactive precipitin are also developed when the Co60B12-binding capacity of intrinsic factor was blocked by unlabeled vitamin B12 and when nonintrinsic factor vitamin B12 binders were substituted for intrinsic factor. In the absence of anti-human IgG in the test system, a precipitin did not form and radioactivity was not demonstrable in the diffusion zone. With antibody-containing serum, intrinsic factor was found in the gastric juice of 12 patients without pernicious anemia and in none of three patients with pernicious anemia.