In 20 iron deficient patients and 21 normal controls the activity of the enzyme δ-ALA dehydratase of erythrocytes was assayed. In addition the urine porphyrins and porphyrin precursor excretions were measured. It was found that in sideropenic patients the erythrocyte δ-ALA dehydratase activity was almost constantly higher than in normals; the difference of the mean values being statistically significant (p < 0.005). A significant diminution of δ-ALA (p < 0.0025) urine excretion was observed, whereas the urine excretion of PBG, CP and UP was found within the normal limits. The results are compared to those reported by other authors.