Seventy skeletal malignancies in 44 dogs were identified among 117 beagles injected as young adults with graded dosages of 0.07 to 104 kBq 241Am kg−1 and maintained for lifetime observation. All of these tumors were osteosarcomas except four fibrosarcomas of bone and four chondrosarcomas of bone. Of these 117 animals, 114 survived beyond the minimum age (of 2.79 y) for radiation-induced bone cancer, and all are now dead. An expression was derived that described the dependence of percent occurrence of bone sarcoma on skeletal radiation dose of A = 0.76 + 30D, where A = percent of dogs with skeletal malignancy within any dosage group, D = average skeletal dose (<3 Gy) at 1 y before death (average skeletal dose was calculated to the presumed start of tumor growth, which we have taken to be 1 y before death), and 0.76 represents the lifetime percent malignant bone tumor response among 132 suitable control dogs in our colony not given any radioactivity. All dosage groups with skeletal doses of >3 Gy at 1 y before death exhibited close to 100% occurrence and appeared to be beyond the region of linearity. Therefore, they were excluded from the derivation of this expression. Similar analysis of corresponding data for beagles given 226Ra as young adults, excluding the two highest dosage groups in which the bone tumor response was 100%, yielded the expression, A = 0.76 + 4.7D, (D < 20 Gy). A ratio of the coefficients in these two expressions indicates the effectiveness at low radiation doses for bone-cancer induction of 241Am relative to 226Ra, or (30 ± 2.6)(4.7 ± 0.47)−1 = 6 ± 0.8. This compares to the relative effectiveness at low radiation doses that was obtained earlier for a 239Pu:226Ra toxicity ratio of about 16 ± 5.