Replication stress is a common feature of solid cancers, and drugs targeting replication stress such as Checkpoint kinase 1 inhibitors (CHK1i) have demonstrated significant preclinical activity especially in combination with replication stress promoting chemotherapies. However, this has not translated into an effective clinical treatment, primarily due to high normal tissue toxicity. We have previously demonstrated that a combination of CHK1i with a subclinical dose of hydroxyurea selectively targets a range of tumour types, importantly with little normal tissue toxicity to even chemo-sensitive tissues. The CHK1i combination also promotes a pro-inflammatory response and immunogenic cell death. In vivo, this drug combination induces tumour regression which is dependent on an adaptive immune response. Here we report the immune responses triggered by this combination in mouse models and demonstrate that this response can be further enhanced by strategies targeting tumour-associated myeloid cells. Immune responses were elicited by the CHK1i combination in all tumour models investigated. The combination enhanced immune responses in tumours that were immunologically “cold”. The common features of the immune responses are increased cytolytic activity and reduced immune suppression in the tumour microenvironment. The responses were dependent on CD8+ T cells. Myeloid cells in the tumour microenvironment were immunosuppressive and this could be reversed by depletion with CSF-1R antibody. Treatment also increased PD-L1 expression on tumour and myeloid cells, and anti-PD-L1, but not anti-PD-1 enhanced survival,. This demonstrates the CHK1i combination is highly selective with minimal normal tissue toxicity, does not adversely affect immune responses, and can trigger an effective anti-tumour immune response in range of tumour settings. Reducing tumour associated myeloid cell number or activity was associated with enhanced anti-tumour immune responses. This work suggests that the myeloid component of tumours may significantly alter treatment responses by suppressing anti-tumour immune activity.