Ovarian cancer is one of the most fatal tumors in women. Due to a lack of symptoms and adequate screening methods, patients are diagnosed at advanced stages with extensive tumor burden (Jelovac and Armstrong, 2011). Interestingly, ovarian cancer metastasis is generally found within the peritoneal cavity rather than other tissues (Lengyel, 2010; Tan et al., 2006). The reason behind this tissue tropism of the peritoneal cavity remains elusive. A prominent feature of this selectivity is ascites, the accumulation of fluid within the peritoneal cavity, containing, amongst others, immune cells, tumor cells and various soluble factors that can be involved in the progression of ovarian cancer (Kipps et al., 2013). The protocol described here is used to isolate mononuclear cells from ascites to study the functionality of the immune system within the peritoneal cavity.