Abstract
The growth characteristics and histological appearance of tumors resulting from transplantation of the tumor lines HEp‐2 and SW480 into pathogen‐free and mouse hepatitis virus infected athymic mice were studied. Subcutaneous or intra‐peritoneal implantation 1 × 10^6^ neoplastic cells into pathogen‐free animals resulted in tumor growth. Subcutaneous transplants grew locally, surrounded by a capsule of connective tissue. The fibrovascular stroma supporting the neoplastic tissue was minimal and infiltration of tumor capsule was observed. Intraperitoneal tumors grew in a multifocal pattern, were not encapsulated, showed marked invasiveness and metastasized. The same number of neoplastic cells (1×10^6^) transplanted into hepatitis‐positive animals failed to develop into grossly visible tumors. When the number of transplanted cells was increased to 2×10^7^, tumors appeared in a few animals. All tumors, regardless of the site of transplantation, were characterized by the presence of severe fibrohistiocytic reaction at the site of implantation that possibly influenced the tumor growth. No evidence supporting T‐cell‐mediated tumor rejection was observed. It is concluded that the state of health of the athymic mice is critical for the growth of human tumors and may account for the variations in reporting successful transplantation of such tumors in nude mice.