Abstract:The effect of interleukin‐1 on iron metabolism in rats was evaluated. Plasma iron decreased from 184 ± 16 μg/dl (mean ± SE) to 24 ± 12 at 6 hours after interleukin‐1 intramuscular administration in non‐fasting rats and 109 ± 6 μg/dl to 12 ± 1 μg/dl in fasting rats, which was significantly lower than in control rats. Ferrokinetic studies showed a more rapid disappearance rate and lower iron turnover in interleukin‐1‐injected rats. The release of iron from the mononuclear phagocyte system to plasma was studied at 3 h after interleukin‐1 administration. Although the percent of radioactivity in plasma of the total injected dose was 3.2 ± 0.6% in interleukin‐1, which was significantly lower than in the control rats (5.4 ± 0.6%) at 9 h after intravenous injection of59Fe chondroitin ferrous sulfate, there was no difference between the amount of59Fe released from the mononuclear phagocyte system over the first 9 h in interleukin‐1 and control rats. These data appear to imply that iron release is unimpaired but that, for some reason, there is an enhanced rate of clearance of the59Fe once it has been released from the mononuclear phagocy