The mechanisms of iron uptake from transferrin and the effects of iron chelators on these processes were investigated in human neuroblastoma cells. This study was performed because numerous reports have indicated that neuroblastoma cells contain iron-rich ferritin and are also especially sensitive to iron chelation by deferoxamine. The mechanisms of iron and transferrin uptake were examined in the human neuroblastoma cell line SK-N-MC by using human transferrin labeled with iodine 125 and iron 59. Internalized and membrane-bound 59Fe and 125I-transferrin were separated with the protease pronase. Total internalized and membrane 125I-transferrin uptake was biphasic with time, whereas total and internalized 59Fe uptake was linear. Iron uptake from transferrin was prevented by incubation at 4 degrees C and also by lysosomotrophic agents. In addition, 59Fe uptake occurred by two processes. The first process was consistent with receptor-mediated endocytosis involving internalization of transferrin bound to specific binding sites. Iron uptake also occurred by a second process, which was not saturable up to a transferrin concentration of 0.06 mg/ml. In terms of quantitative iron uptake, however, the second process was far less important than receptor-mediated endocytosis. Deferoxamine (0.25 mmol/L) only slightly increased 59Fe release from prelabeled cells; the orally effective iron chelator pyridoxal isonicotinoyl hydrazone (0.25 mmol/L) was six times more effective. Moreover, when pyridoxal isonicotinoyl hydrazone (0.2 mmol/L) was added together with labeled transferrin over a 2-hour incubation, 59Fe uptake from transferrin decreased to 18% of the control value, whereas deferoxamine (0.2 mmol/L) had no appreciable effect. Even though deferoxamine (0.1 mmol/L) had little effect on 59Fe uptake or release, it reduced uptake of tritiated thymidine to 33% of the control value after a 24-hour incubation. Three analogs of pyridoxal isonicotinoyl hydrazone, pyridoxal benzoyl hydrazone (#101), pyridoxal p-methoxybenzoyl hydrazone (#107), and pyridoxal m-fluorobenzoyl hydrazone (#109), had chelation activities comparable to that of pyridoxal isonicotinoyl hydrazone and were more effective than either deferoxamine or pyridoxal isonicotinoyl hydrazone at preventing tritiated thymidine uptake. These results suggest that the pyridoxal isonicotinoyl hydrazone analogs have potential as effective antiproliferative agents and deserve further investigation.