1. The effect of chloramphenicol on Fe uptake and Fe incorporation, using Fe59, was studied in human bone-marrow cultures.
2. Choramphenicol, in a concentration of 500 µg. per ml., resulted in a slight depression of uptake of iron by the erythroid cells and in a marked depression of the incorporation of iron into heme.
3. The results suggest that one toxic effect of chloramphenicol upon the bone marrow may involve direct interference with heme synthesis.
4. Under certain experimental conditions the inhibitory effect of chloramphenicol upon iron uptake and heme synthesis by the erythroblast is reversible.