concentration and therefore the dose of radiation cannot be accurately calculated. The method of approach adopted was to compare in animals the toxic effect of a particular isotope to that of radium and to use these data for establishing the permissible dose in man equivalent to the 0.1 microgram of radium which is the accepted maximum permissible dose. Radium deposition in humang has occurred in hazardous amounts and several studies have been made to estimate the lowest levels which will cause damage. The Bureau of Standards in 1941 set 0.1 microgram as the maximum permissible level of body burden for radium. This was based on a series of cases where death had occurred with a level of radium as low as 1.2 micrograms. Since the maximum permissible level embodies a safety factor of about 10, the level of 0.1 microgram was established. The radium preparations to which the above cases were exposed also contained other radioactive materials, principally mesothoriun. Since the mesothorium component was not estimated, it was felt that the damage attributed to the calculated radium levels must be due in part to the mesothorium and therefore the radium standard would have been set too low. In order to investigate this problem, W. B. Looney22/ has compared the cases involved in the mesothorium-radium series with a series of patients given radium preparations as a form of medication. His studies suggest that in the two series of cases there is a threshold level for detectable disease at about 0.5 - 1.0 microgram of radium firmly deposited in the skeleton. Within the limits of accuracy of measuring the response to various levels of radium no increased effect could be noted due to the mesothorium content of the patients exposed to the mixture. From the viewpoint originally set for the concept of "Maximum Permissible Levels", Looney's study indicates that the value 39/ Looney, W.B., U.S. Naval Hospital, National Naval Medical Center, Bethesda, Maryland, 1955, Personal Communication. 99