There are MPC values for many additional radionuclides in the revised handbooks (now ready to go to press) of the National Committee on Radiation Protection, NCRP, and of the International Commission on Radiological Protection, ICRP. The basic philosophy and principal standards used in obtaining these MPC values have not changed. The MPC values are calculated on the basis of a dose rate of 0.6 rem/week to the thyroid or skin, 0.56 rem/week to bone, 0.1 rem/week to the gonads cr total body, and 0.3 rem/week to other organs of the body. In this revised edition the period of occupational exposure is 50 years (it was formerly 70 years). The GI tract is the critical body organ for more than 50 per cent of the 230 radionuclides listed in these publications. All WPC values are calculated on the assumption that the body burden of the radionuclides is an exponential function of time. In addition, in the few cases where sufficient data are available, calculations are made on the basis of a power function of time.