This thesis describes the observations which led to the development of methods for both the assay of human gastric intrinsic factor, and for the detection of antibodies against intrinsic factor. Studies on the secretion of intrinsic factor in healthy subjects, and in patients with megelobiastic anaemia and disorders of the gastric mucosa, are presented. The effect of various pharmacological agents on the secretion of intrinsic factor is described, and the output of intrinsic factor is related to the secretion of hydrochloric acid and pepsin. Finally the relevant literature is reviewed, and the work described herein is discussed in relation to the observations of other investigators in this field.