1. The vitamin B12 content of the erythrocytes from 20 normal men has been found to vary from 135 to 230μμg per ml of the packed erythrocytes with a mean value of 195μμg. 2. Vitamin B12 in human erythrocytes exists in the hemoglobin fraction in some combined form with protein which has an electrophoretic mobility similar to that of hemoglobin. 3. Heat stability in pH 6.8 phosphate buffer of the combined vitamin B12 in the hemoglobin fraction has been investigated. Release by heat of the combined vitamin B12 takes place more readily in pH 4.6 acetate buffer than in pH 6.8 phosphate buffer, although the release by 30 minutes heating in both buffers is almost equal. 4. The solution of the crystallized human oxyhemoglobin has a limited capacity to bind the added vitamin in vitro. The added vitamin which is bound in the hemoglobin solution can be recovered from the hemoglobin band on paper strips by a paper electrophoretic technique. 5. From the results it is suggested that vitamin B12 in human erythrocytes exists in a combined form with hemoglobin.