We show that in disordered metallic systems, spin-orbit interactions lead to the existence of two metallic phases separated in temperature. The low-temperature metallic phase arises from spin-orbit interactions. The crossover temperature is proportional to (Z-Z′)2(Zis the atomic number of the donor andZ′that of the matrix) and is expected to be observable for donors with large atomic numbers. The presence of two metallic phases implies two mobility edges, existing at different temperatures, and affecting the conductivity when the Fermi energyEFlies below the mobility edgeEcWe thus predict anomalies in the conductivity in this range of concentration, consistent with the measurements of Long and Pepper on Si:Sb.