The effect of iron absorption on subsequent hemoglobin regeneration was investigated by repletion and subsequent analysis of an anemic weanling rat model. Primary assay procedure in these studies was the Regeneration Efficiency method. As repletion or curative assay techniques form the central body of investigative research in iron utilization, the first subject of inquiry was the effect of repletion interval and degree of depletion upon the animals as monitored by physical indicators such as blood volume, growth and hematinic response. Percent blood volume is a particularly useful figure in the calculation of the amount of hemoglobin iron but was found to be relatively constant in a preliminary experiment. Accordingly, the effects of growth and anemia on hemoglobin response and blood volume were examined in 122 weanling male rats which had been depleted for seven days by low iron diet and phlebotomy, then repleted by feeding iron rich diet (47.1 ppm) for zero, five, 10 and 15 days. Percent blood volume proved to be rather constant at 7.5 in these pre-pubescent animals. The most severely depleted animals appeared to regenerate hemoglobin initially rather than replenish tissue iron. Regeneration Efficiency and AOAC assay methods were compared in a study involving the relative potency of two iron salts, ferrous sulfate and ferric orthophosphate, and three commercial cereal sources containing electrolytically reduced, hydrogen reduced or ferric orthophosphate supplements. These two assays utilized 202 albino male weanling rats. The superior availability of ferrous sulfate was evident in both regeneration efficiency and AOAC assays, as was the relative similarity of the dietary iron sources, yet the commercial sources in general provided more available iron in comparison to the reference ferrous sulfate than expected, possibly due to the influence of dietary components and processing variables. Bioavailability based on dietary iron concentration response correlated highly (r = 0.94) between animal groups analyzed using the Regeneration Efficiency and AOAC methods. Stress may have been a factor in animals assayed by the AOAC technique which used a 28 day depletion interval followed by a two week repletion. The Regeneration Efficiency method, which utilizes shorter depletion and repletion intervals also offered sufficient amounts of dietary iron for normal growth. The calculation of iron bioavailability also is dependent on such pertinent variables as dietary intake amount, body weight gain and percent blood volume.