Replication stress is a common feature in most cancers, including melanoma and high-grade serous ovarian cancer. We have previously demonstrated that elevated replication stress can be selectively targeted using a combination of the cell cycle Checkpoint kinase 1 inhibitor and low-dose hydroxyurea. This combination treatment promotes tumour selective killing with little normal tissue toxicity even in chemo-sensitive tissues, which should translate to reduced impact on patients’ quality of life. Extensive preclinical data have been generated for this combination in melanoma, where the treatment promotes pro-inflammatory cytokine expression in treated tumour cells and anti-tumour immune response consisting of innate and adaptive immune components. Interestingly, in two syngeneic mouse melanoma models, the most significant changes were increased NK and NKT cell infiltration while there was little change in CD8+ T cell numbers or activity. The size and significance of the NK and NKT cell response suggests that these immune cell types functionally contribute to the response observed. The macrophage content of the tumours may also influence the treatment-induced immune response. Noticeably, our combination treatment was effective in blocking tumour growth in a syngeneic mouse ovarian cancer model as well. Again, NK and NKT cell responses were observed, but in this model, there was concurrent down-regulation of immunosuppressive signals that are common features of ovarian cancer tumour immune microenvironment. Using an ex vivo model developed from ovarian cancer patient-derived ascites to assess tumour cell and immune responses to our combination treatment, we observed that CD3+ lymphocytes clustered around the tumour cells and HLA-DR+ CD19+ B cells accumulated in the culture supernatant. These studies demonstrate the effectiveness of our treatment in promoting anti-tumour immune responses. Understanding these responses will provide an opportunity to combine with other immunotherapies to enhance the immune response observed.