HeLa cells were heterotransplanted to nude mice and their response to some of antitumor agents was investigated. HeLa cell-tumor readily grew in nude mice and no regression was observed. Metastases to the lung and other organs were noticed in some of the animals. Histopathological examination revealed that the tumor retained the original characteristics of human epidermoid carcinoma. Standardization of HeLa cell-tumor in nude mice for the screening and evaluation of antitumor chemotherapeutics was attempted. Marked inhibition of tumor growth was observed with lower doses of Mitomycin-C, 5-fluorouracil, Adriamycin, and Bleomycin. The tumor regression was observed with high doses of Mitomycin-C, Adriamycin, and 5-fluorouracil. However, cyclophosphamide, cytosine arabinoside, and daunorubicin had little effect on the tumor growth. Complete regression was not obtained with any of the test agents and active regrowth took place even with the most effective compound. Considerable variation in the effect on tumor growth was observed among the test compounds, while histopathological findings were much alike; few mitotic figures, vacuolization, and pyknosis were main changes in tumor cells, and large foci of necrosis and hemorrhage were present in the degenerative areas. The regrowth was initiated around the capillaries in the necrotic tumor tissue.