In previous studies1 in this laboratory it was observed that the retention of iron by the infant decreased noticeably when the vitamin B1 content of the diet was increased by supplements of wheat germ. The object of the work to be reported here was to determine whether or not the lowered retention of iron was due to a specific effect of the vitamin. The iron retention and hemoglobin formation found for a normal infant during periods in which the diet was supplemented with a solution of crystalline vitamin B1 have been compared with those found in control periods during which no vitamin B1 supplement was given. This comparison has been made at three levels of intake of iron in order to observe both the effect of the vitamin at different levels of iron intake and the effect of the different levels of intake on the retention