We have shown that multiple macrophage populations are associated with prostate cancer (PC) bone metastases: resident macrophages in adjacent bone marrow (BM), recruited macrophages accumulated at the tumour periphery, osteal macrophages in the woven bone response and tumour associated macrophages (TAMs) embedded with the tumour bulk. Because of evidence implicating macrophages in both pro- and anti-tumour functions, and the importance of tumour-microenvironment interactions in sustaining tumour growth and immune evasion, we hypothesize that targeted depletion of specific macrophage subsets has potential to disrupt PC associated pathology. Granulocyte colony-stimulating factor (G-CSF) is clinically used to treat neutropenia subsequent to chemotherapy in PC and we have also demonstrated G-CSF treatment in wild type mice depletes at least a portion of BM macrophages. Therefore we explored the impact of G-CSF treatment on PC-bone/BM microenvironment interactions using our established immune-competent mouse model of PC growth in bone. Mouse RM1(BM) cells were seeded into right tibiae by intra-tibial injection (IT) and contra-lateral tibiae were injected with saline. Bi-daily G-CSF (125 µg/kg) or saline treatment was initiated 1 day after IT injection and continued for 6 consecutive days. Day 7 post IT, saline injected contralateral BM was collected for flow cytometry and tumour bearing tibiae were analyzed by quantitative histology. Flow cytometry demonstrate that G-CSF significantly depleted two specific BM macrophage subsets: F4/80⁺Ly6G⁺ erythroid island and F4/80⁺Ly6G^-VCAM1⁺CD169⁺Ly6C^- resident macrophages. Interestingly, within G-CSF treated tumour-bearing limbs, F4/80⁺ resident macrophages in adjacent BM were reduced but TAMs were minimally affected. Tumour-associated woven bone formation was significantly reduced in G-CSF treated animals compared to controls. Impact on tumour growth is currently being examined.  Overall, these observation support that CD169⁺ macrophage mediated woven bone response occurs through tumour-mediated subversion of normal resident BM macrophage function, specifically osteal macrophages, rather than a de novo pathological mechanism driven by TAMs.