SummaryThe absorption of radioactively labelled iron added to milk given to children less than 3 months old was investigated. The percentage of absorption of the tracer was found to be considerably larger than that observed at a later age. A comparison with figures for net retention of iron from milk in children of similar age indicates that excretion of iron takes place in these infants, or that true absorption of iron is not measured by the tracer method. Some difficulties in the interpretation of the conventional tracer absorption test are indicated. The phenomenon of “mucosal block” for iron is present also in the present age group. It is pointed out that simple physico‐chemical phenomena might explain all data which have led to the concept of the “mucosal block