Electrical resistivity (&rgr;) of samples with different grain sizes, made from iron of three different purities (containing a maximum of 34, 79, and 2150 atomic ppm impurities, respectively), has been studied at 4.2°K in the presence of applied longitudinal magnetic fields up to 60 kOe. It has been found that the resistivity is influenced not only by impurities and magnetic domain arrangements but also by the grain‐boundary area per unit volume. The ratioR=&rgr;(300.0°K)/&rgr;min(4.2°K) varies from 2875 to 528 for the interfacial grain area range of 0 (single crystal) to 7.0 mm−1in the highest purity iron. (&rgr;minis the minimum in the resistivity measured as a function of the applied field at 4.2°K.) Due consideration of these large magnetic and grain‐boundary effects discloses a clearer correlation betweenRand purity.