A radioimmunoassay for human intrinsic factor (IF) is presented based on the competitive reaction between unabeled IF and IF labeled with radio B12 for the binding sites of a specific anti-IF antibody found in the serum of a patient with pernicious anemia. Since purified human IF was not yet available, the standard curve used for reference was made by as-saying dilutions of pooled normal gastric juice. The posthistamine gastric juice from a group of normal subjects, patients with histamine fast achlorhydria, and patients with pernicious anemia had IF levels of 32 to 179%, 12 to 63% and 0 to 6.6%, respectively. Boiling the gastric juice destroyed most of the immunologically reactive IF. Blood group substances A and B did not interfere in this immunoas-say. Hog IF cross-reacted with human antihuman IF antibody. One sample of human bile assayed had no IF.