Extramedullary hematopoiesis is of special interest to physicians because of its relation to hematologic disease. Because it normally remains asymptomatic, sites are typically found by chance. Effective diagnosis involves a specific, reliable, whole-body and low-cost method of screening. Although radiologic methods such as computed tomography and magnetic resonance imaging can only suggest the presence of extramedullary hematopoiesis, apart from invasive and therefore risky biopsy procedures, only scintigraphy can detect and confirm the nature of hematopoietic tissue. Although radioactive tracers commonly in use partly lack the demands for specific diagnosis, Tc-99m-labeled antibodies (NCA-95) seem to combine the advantages of different scintigraphic approaches. Two patients with dyserythropoetic anemia and paravertebrally situated pelvic and thoracic tumor masses were studied for extramedullary hematopoiesis. Planar and SPECT images were obtained 6 and 24 hours after injection of 800 to 850 MBq (22 to 23 mCi) Tc-99m-labeled monoclonal antibodies (BW 250/183). In both patients, tracer accumulated in the masses, thereby revealing hematopoietic tissue. Biopsy confirmed these findings. By using Tc-99m-labeled monoclonal antibodies to detect extramedullary hematopoiesis, the demands of diagnosis were met. As an alternative to invasive diagnostic procedures, this tracer combines the advantages of other radioactive substances previously used, such as radioiron, In-111 chloride, and Tc-99m colloids. This low-cost agent is readily available and when applied, reliable, and delivers whole-body images free of additional uptake in the liver or spleen.