Radiologic investigation of concealed gastro-intestinal hemorrhage with 51Cr was conducted for its detection and quantitative estimation in 102 patients with iron-deficiency anemia (IDA), loss of blood from the digestive tract was detected in 52 of them. A definite relationship was noted between the volume of hemorrhage and the disease responsible for the development of IDA. Moderate hemorrhage from 2.5 to 15.0 ml/day was detected in chronic inflammatory diseases of the gastro-intestinal tract (GIT): chronic gastritis, colitis. High volumes of hemorrhage are characteristic of diseases attended by gross changes in the digestive tract: polyps, tumors, diverticula, varicosis in GIT. It has been shown that the radiologic method of quantitative estimation of concealed gastro-intestinal hemorrhage with the use of 51Cr is highly sensitive and accurate, that permitted the authors to recommend this method for both practical and scientific investigations.