We evaluated the levels of CSA in the serum of and in the medium conditioned by marrow-free femurs of 12 to 16-week-old female CAF1 mice whose marrows had been ablated with the bone-seeking radionuclide, 89Sr. Intact mice were studied 10 to 56 days after 89Sr injection, and mice splenectomized on days 14 and 42 after injection of 89Sr were studied on days 21 and 56, respectively. Control mice were injected with cold 89Sr; sham-splenectomized mice were used when appropriate. None of the mice had any significant levels of CSA in the serum, even the leukopenic splenectomized 89Sr-treated mice. Femur-conditioned medium from all groups contained sizable, but approximately equal, amounts of CSA; thus 89Sr marrow ablation did not adversely affect the capacity of femurs to elaborate CSA. Intraperitoneal injection of endotoxin effected an increase in serum CSA in both intact and splenectomized 89Sr marrow-ablated mice which was equal to that found in the control mice.