The mammary gland undergoes development and regression over the course of the ovarian cycle under the regulation of ovarian hormones. Macrophages are implicated as local mediators of this tissue remodeling and may also affect immune surveillance and tumor incidence. To investigate cycle-related changes in macrophage phenotype, mammary gland cells from naturally cycling Cfms-Gfp mice recovered at estrus, metestrus, diestrus, and proestrus were analyzed by flow cytometry. Macrophage expression of MHCII was highest in the proestrus phase, with a 1.6-fold increase compared to the metestrus phase. Similarly, macrophage expression of CD204 was 1.9-fold higher at proestrus compared to estrus. Conversely, macrophage expression of NKG2D was increased at metestrus and diestrus by 7-fold and 5-fold, respectively, compared to estrus. To investigate hormonal regulation of macrophage phenotype, an ovariectomy and hormone replacement model was utilized. Ovariectomized mice were stimulated with exogenous estradiol and progesterone to induce early alveolar development, then given progesterone receptor antagonist RU486 to elicit alveolar bud regression. Progesterone and estradiol in combination reduced macrophage expression of MHCII and CD204 by 5-fold and 3-fold, respectively, and increased macrophage expression of NKG2D by 4-fold. Administration of RU486, following estradiol and progesterone, reversed the macrophage phenotype. These results reveal an essential requirement for ovarian hormones in regulating macrophage phenotype in the mammary gland and indicate that progesterone is particularly critical for controlling macrophage antigen presentation and immune surveillance capacity.