Leukocytes of the macrophage lineage are abundant in the ovarian tissues and have an important function in both follicular development and regression of postovulatory follicles. In this study, we tested the hypothesis that continuous production of macrophages in the ovarian stroma is maintained by a resident population of progenitors. We established a long-term culture of ovarian follicular stromal cells from BALB/c and green fluorescent protein-transgenic (GFP-TG) C57BL/6 mice. Nonadherent cells were collected and tested for hematopoietic function in vitro and in vivo. Histological and ultrastructural analyses revealed a homogenous population of monocyte-like rounded cells. Nonadherent cells continued to proliferate in culture for several months without senescence. When plated at very low density in methylcellulose, these cells formed colonies consisting of monocyte-like cells. Ovarian monocyte-like cells reacted with CD45, CD11b, CD11c, and Ly6-Gr-1 cell surface markers. A distinct CD45low population within these cells reacted with CD117 (C-kit) surface marker, suggestive of a primitive hematopoietic progenitor. Fifty thousand nonadherent cells failed to provide radioprotection to lethally irradiated mice and thus were not considered to be equivalent to pluripotent hematopoietic stem cells. Ovarian nonadherent stromal cells were positive for alkaline phosphatase but lacked embryonic cell antigens stage-specific embryonic antigen (SSEA-1) and Oct-4. We conclude that in the ovaries, a higher requirement for macrophages is provided by a resident stromal population of progenitors whose progeny is restricted to the production of cells of the monocyte-macrophage lineage.