Myelosuppression is a common dose limiting toxicity of cytotoxic oncology drugs. Nanoparticles may distribute to bone marrow and/or release drug that distributes to bone marrow. Therefore, understanding potential toxicity of nanoparticles or drugs which nanoparticles carry is an important step in preclinical safety evaluation. This document describes a protocol for quantitative analysis of granulocyte-macrophagecolony-forming units, employing murine bone marrow (BM).