Soft tissue sarcomas (STS) are a commonly encountered cancer in dogs. Local recurrence of the tumor is a risk for tumors incompletely removed by surgery. Local recurrence and subsequent treatment can have a significant impact on dogs and owners. In addition, dogs with advanced STS or STS that have spread elsewhere in the body have limited effective treatments. Immune cells within STS are prognostic and a drug target in humans. There is an overall lack in the assessment of immune cells infiltrating STS in dogs. We have performed preliminary experiments for several immune cell markers and have identified immune cells within dog STS. Initial results demonstrate that macrophages, a specific immune cell type, are common. The proposed study aims to characterize immune cells within STS and to assess if macrophages are a prognostic factor for disease progression. Based on human studies, we hypothesize that at least 50% of canine STS will be predominated by immunosuppressive macrophages rather than other immune cell types. Due to potential immunosuppressive characteristics of some macrophages, we also hypothesize that dogs with increased immunosuppressive macrophages will be more likely to have disease recurrence or spread. Our findings would identify potential prognostic factors for STS in dogs and targets for new therapies as well as progress our understanding of the disease process. This information would advance the care of dogs with STS.