Mice with a naturally occurring Csfm(op)/Csfm(op) (op/op) gene mutation lack functional macrophage-colony stimulating factor (M-CSF) and are deficient of M-CSF-derived macrophages. They are severely monocytopenic, and their remaining M-CSF-independent macrophages were shown to differ in differentiation and distinct functions when compared with phenotypically normal mice of the same background. It is not known if osteopetrosis mice (op/op mice) are able to mount a specific immune response against intracellular pathogens, as this would require complex effector functions by macrophages. We therefore investigated the ability of op/op mice and their M-CSF-independent macrophages to combat infection with Leishmania major. op/op mice retained the ability to resist an infection with L. major by mounting a T helper cell type 1 cell response, eliminating parasites and resolving the lesions. Macrophages from op/op mice were able to sufficiently perform effector functions in vitro, such as phagocytosis, production of leishmanicidal nitric oxide (NO), killing of parasites, and release of interleukin (IL)-12. There were quantitative differences, as M-CSF-derived macrophages from hematopoietic organs of control mice showed significantly higher rates of phagocytosis and higher NO release after stimulation with lipopolysaccharides than corresponding macrophages from op/op mice. In contrast, when peritoneally elicited macrophages were used, those from op/op mice revealed a stronger response than those from control mice with regard to release of NO or IL-12. These differences suggest that M-CSF-independent maturation of op/op monocytes subsequent to their release from hematopoietic tissue exerts influence on their effector functions. However, M-CSF or M-CSF-derived macrophages are not necessary for an effective immune response against L. major.