In this research, we optimized parameters for xenotransplanting WM-266-4, a metastatic melanoma cell line, including zebrafish site and stage for transplantation, number of cells, injection method, and zebrafish incubation temperature. Melanoma cells proliferated, migrated and formed masses in vivo. We transplanted two additional cancer cell lines, SW620, a colorectal cancer cell line, and FG CAS/Crk, a pancreatic cancer cell line and these human cancers also formed masses in zebrafish. We also transplanted CCD-1092Sk, a human fibroblast cell line established from normal foreskin and this cell line migrated, but did not proliferate or form masses. We quantified the number of proliferating melanoma and normal skin fibroblasts by dissociating xenotransplant zebrafish, dispensing an aliquot of CM-DiI labeled human cells from each zebrafish onto a hemocytometer slide and then visually counting the number of fluorescently labeled cancer cells. Since zebrafish are transparent until approximately 30 dpf, the interaction of labeled melanoma cells and zebrafish endothelial cells (EC) can be visualized by whole-mount immunochemical staining. After staining with Phy-V, a mouse anti-zebrafish monoclonal antibody (mAb) that specifically labels activated EC and angioblasts, using immunohistology and 2-photon microscopy, we observed activated zebrafish EC embedded in human melanoma cell masses. The zebrafish model offers a rapid efficient approach for assessing human cancer cells at various stages of tumorigenesis.