Multiple myeloma cells are dependent on the bone marrow environment for survival, and thus the specific molecular interactions involved represent attractive targets for the development of new treatment approaches. We have found that myeloma cells express a receptor call CD28 that appears to be involved in these pro-survival interactions. We proposed to test whether antibodies against CD28 can inhibit the growth of multiple myeloma in a mouse model, and what the side effects of this kind of treatment are.