Cytotoxic chemotherapy is often complicated by hematopoietic toxicity. The degree of aplasia and the rapidity of count recovery following chemotherapy are indicative of bone marrow reserve. Patients who generally have a normal bone marrow function will recover from chemotherapy-induced cytopenia relatively rapidly. In contrast, patients that have poor bone marrow reserve will have significantly prolonged period of aplasia. Predicting the hematopoietic toxicity of radioimmunotherapy is an important dosimetry consideration. Unfortunately, there are no good models for predicting toxicity from chemotherapy that could be applied to radioimmunotherapy. However, models used to predict the ability to harvest autologous stem cells for use after high dose chemotherapy may be useful in predicting bone marrow reserve and potential toxicity from radioimmunotherapy. These models indicate that the successful mobilization of stem cells into the peripheral blood is inversely proportional to exposure to stem cell toxic drugs. Establishing criteria that will help predict the amount of myelotoxicity sustained from radioimmunotherapy could lead to improved dosimetry and ultimately to better therapy for patients.