This study was initiated when a well known therapeutic radiologist expressed the opinion that “the bone-sparing effect of cobalt 60 is a ‘spicule’ of the radiation therapist's imagination.” The argument that supervoltage radiation has a “bone-sparing” effect was adduced because of physical principles. Since then it has been questioned by numerous authors (1, 2), some maintaining that it is nullified by the lower biological effectiveness of supervoltage (1). Garland suggests the biological effectiveness of cobalt 60 is a factor of 0.8 of orthovoltage. It has been shown that roentgen rays of a h.v.l. of 0.4 mm. Cu produced twice the damage in cortical bone as roentgen rays of 2.4 mm. Cu, in terms of osteocyte destruction (Rosenthal and Marvin, 3). The same authors were unable to demonstrate differences in stunting of bone growth between the softer and harder radiation and this was attributed to cartilage being of soft-tissue density. In this attempt to compare cobalt-60 and 250-kv radiation we utilized the ...