may be illustrated by one experiment using swine where, the LD 50/30 values for external dose decreased from 500 to 350-400 roentgens when the exposure was changed from unilateral to bilateral (the radiation 7 exposure was first on one side only, then from opposite sides of the | t subject) .22 With a fallout field, the source probably would be more radial, thus a "roentgen'tt as measured in air would have more biological effect than one where the source is unilateral such as from the immediate radiations at the instant of a burst (although there is some scattered radiation), or from X-ray machines which have been used frequently with unilateral beams in developing data on biological effects of radiation. 5. Biological repair factor. It has been recognized that, in general, the longer the period over which a given radiation dose is delivered, the less is the resultant biological effect, except for such aspects as the genetic effects and life shortening. In situations of heavy fallout and relatively large potential radiation doses, the biological repair factor may be considered in estimating incapacitating and lethal doses. Since past experiments usually have been designed for other purposes, the data from these do not readily elucidate the rate of repair or the proportions of reparable and irreparable damage resulting from differently timed doses. Varying relationships have been demonstrated, depending upon the species or even the strain of animal, as well as the criteria selected for study, such as skin damage, life shortening, ant and LD 50 values. ae Our present knowledge does not permit establishment s -10-