In a mouse you'll find lyphocytes (B cells and T cells), macrophages and fibroblasts among other cells. Here is a link to some more detailed info: 

http://www.jove.com/video/1488/isolation-of-mouse-peritoneal-cavity-cells

Thank you Ellie!! 

depend on from where you aretaking the peritoneal fluid..if its from human peritoneal cavity its mainly mesothelial cell and few lymphocytes and rbc.Also it depends on the reason for opening peritoneal cavity.If its for cancer surgery you can expect to find malignant cells aa well