Using the most recent evaluations of power loss by impurity radiation, we have calculated the maximum permitted impurity concentration for various species as a function of Q, the ratio of deuterium-tritium (D-T) fusion power to injected beam power. These criteria for maximum impurity concentration must be satisfied before applying the usual neτE versus Ti conditions for obtaining a given Q value. For ,l the critical impurity concentration fcz varies as Z−2.2 to −2.5. The tolerable concentration of medium- and high-Z impurities for operation at low can be at least one order of magnitude larger than the concentration allowed for ignition, provided that the plasma temperature is maintained by reacting ion beams.