Bone marrow is the primary dose-limiting organ in radioimmunotherapy. Athymic nude mouse models are used to guide radioimmunotherapy in humans. In the mouse, the dimensions of the marrow are comparable to the mean range of the beta particles for a wide variety of beta-emitting radionuclides, so local beta energy deposition cannot be assumed. Methods: We have developed a computer simulation model in which slab, spherical and cylindrical geometries of the bone marrow of the mouse were incorporated. The energy deposition within the marrow was estimated using beta dose point kernels for several beta-emitting radionuclides. Results: The calculated percentages of energy deposited in the mouse marrow using the full geometry were 46%, 24% and 10% for 131l-, 186Re- and 90y-radiolabeled antibodies, respectively. Assuming a concentration of activity in the marrow of 0.36 times the blood activity concentration, the percentages of energy deposition in the marrow from marrow and whole-body sources were 61%, 40% and 29% for 131, 186Re and 90Y, respectively. Conclusion: This work shows that, even for the lower mean beta energy-emitting radionuclide, such as 131l, accurate computation of the mouse bone marrow dose involves including both the energy loss from beta decays within the marrow and dose contributions from tissue surrounding the marrow.