INTRODUCTIONBone marrow-derived macrophages (BMM) are primary macrophage cells, derived from bone marrow cells in vitro in the presence of growth factors. Macrophage colony-stimulating factor (M-CSF) is a lineage-specific growth factor that is responsible for the proliferation and differentiation of committed myeloid progenitors into cells of the macrophage/monocyte lineage. Mice lacking functional M-CSF are deficient in macrophages and osteoclasts and suffer from osteopetrosis. In this protocol, bone marrow cells are grown in culture dishes in the presence of M-CSF, which is secreted by L929 cells and is used in the form of L929-conditioned medium. Under these conditions, the bone marrow monocyte/macrophage progenitors will proliferate and differentiate into a homogenous population of mature BMMs. The efficiency of the differentiation is assessed using fluorescence-activated cell sorting (FACS) analysis of Mac-1 and 4/80 surface antigen expression. Once differentiated, the BMMs are suitable for numerous types of experimental manipulations, including morphological, gene expression, and physiological studies. For example, phagocytic cells such as macrophages have a unique ability to ingest microbes. We describe a test for the phagocytic efficiency of BMMs by exposing them to fluorescently labeled yeast zymosan bioparticles. Also, a method to deliver DNA or small interfering RNAs (siRNAs) into these hard-to-transfect cells is described. Finally, the proliferation of the BMMs is assayed using carboxyfluorescein succinimidyl ester (CFSE), a fluorescein derivative that partitions equally between daughter cells after cell division.