Feline vaccine-associated injection site sarcomas (ISSs) were first reported in 1991. Although radical surgery, radiation, and chemotherapy are all used to treat these aggressive tumors, recurrence is common. Nanoparticles can provide building blocks for therapeutic mechanisms and increase ability to visualize tissue. Gold nanoparticles (AuNP) can increase the radiation dose deposited into tissue. This in vitro study evaluated the effect of AuNP on ISS cytotoxicity and colony formation. Although AuNP alone did not apparently alter short-term viability or cell cycle of ISS cells, cellular proliferation decreased significantly. Thus, AuNP shows initial promise as a long-term therapeutic to decrease ISS growth.