Although the transplantation of human neoplasms in immunodeficient mice is now a well-established procedure, the majority of primary malignancies cannot be successfully maintained for long periods of time in adult athymic (nude) and asplenic-athymic (lasat) mice. Various lipids such as cholesterol, cholesterol oleate, stearic and palmitic acid esters markedly depress the RES phagocytic activity and immunocompetence of mammals. In view of the immunosuppressive properties of certain lipids and in order to graft and grow as many tumors as possible, further studies into the effects of lipids on the growth of heterotransplanted human tumors is warranted. Lipids may enhance local growth and facilitate the development of metastases rarely seen in nude and lasat mice bearing xenogeneic cancer cells. Lipids may accelerate human malignant cell proliferation in mice by both depressing further the defense of host and modifying the cancer cell membrane. The relationship of lipids to the onset and progression of 'spontaneous' tumors in humans is not known.