Blood donation at the accepted frequency of one donation every 8 weeks is generally regarded as excessive for preservation of donor iron reserves, especially in women. A double-blind study was conducted of 627 individuals who had previously donated blood to determine the effect of iron supplementation on iron reserves. Half the donors received ferrous fumarate at a dosage of 66.6 mg elemental iron daily for 45 days. The other half received placebo. Eight blood variables reflecting the iron status of the donors were determined at the time of the initial donation and again 2 months later at the time of their second donation. Statistical analysis of the blood variables indicated that the prescribed amount of ferrous fumarate should be considered the minimal effective dose for maintenance of reduced iron storage in blood donors. However, the data are furnished for the analysis of panels of experts for any conclusions and recommendations concerning this study. Feostat, the iron supplement, was tolerated well by the participating donors. Gastrointestinal upset occurred in eight donors receiving Feostat and in 10 donors receiving placebo.