A research team has developed a novel method to detect hemangiosarcoma (HSA) in large breed dogs in its early stages by looking for cell markers specific to malignant endothelial cells. The test can be applied as a blood test to all commonly effected breeds at their annual checkup. This would allow for early diagnosis that can open the door for treatment options that were formerly impossible. The detection method shows that HSA is a tumor of “primitive” endothelial cells and that these can be distinguished from “normal” or “benign” proliferative endothelial cells by the concurrent expression of three different cell surface proteins. These proteins can be detected using multi-parameter flow cytometry. The assay also allows for the diagnosis of leukemia and/or lymphoma, which can express some of the same proteins. The ability to distinguish between hemangiosarcomas and angiosarcomas, and leukemia and lymphomas means that treatments can be tailored to the particular disease, improving the efficacy of treatment in canines and humans.