1. 1. Rabbit reticulocytes with a high level of non-heme radioiron induced by preincubation with isonicotinic acid hydrazide and transferrin-bound 59Fe, were reincubated with various synthetic chelating agents and the amount of radioiron released from the cells was determined. Some substances, especially derivatives of pyridoxal or 2-hydroxybenzaldehyde and isonicotinic acid hydrazide or benzhydrazide, were found to mobilize significantly iron from 59Fe-labeled reticulocytes. The effectiveness of the compounds tested decreases in the following order: pyridoxal isonicotinoyl hydrazone, pyridoxal benzoyl hydrazone, 2-hydroxybenzal isonicotinoyl hydrazone, 2-hydroxybenzal benzoyl hydrazone, pyridoxal-valine Schiff base, pyridoxal. The efficiency of these compounds exceeded the ability of common chelators such as desferrioxamine, 2,2′-bipyridine, nitrolotriacetic acid, etc., to mobilize iron from reticulocytes. 2. 2. Iron mobilization from reticulocytes by pyridoxal isonicotinoyl hydrazone requires ATP to be produced by cells and is completely blocked by low temperature (4°C). Although the effect of desferrioxamine is also prevented by low temperature, modest iron mobilization due to this chelator seems to occur independently of ATP production in reticulocytes. 3. 3. Pyridoxal isonicotinoyl hydrazone mobilizes iron mainly from mitochondria and in part also from ferritin. Although 2,2′-bipyridine seems to enter reticulocyte mitochondria and bind iron there, this chelator is not able to release iron either from mitochondria or from the cells. 4. 4. Reticulocytes with a high level of non-heme radioiron are envisaged as a useful system for testing biological effectiveness of various iron chelators. 5. 5. Pyridoxal isonicotinoyl hydrazone was shown to be an effective in vivo chelator since its administration to mice decreased 59Fe radioactivity in liver, spleen and kidney.