Transplantable animal tumors have been associated with the discovery of most clinically active anticancer agents. They are still useful today in conducting detailed evaluations of new candidate anticancer drugs. Three protocols relating to transplantable experimental tumors are described in this unit. Included are the intravenously‐implanted murine P388 leukemia, the subcutaneously‐implanted murine B16 melanoma and two examples of subcutaneously‐implanted human tumor xenografts, the LX‐1 (lung) and MX‐1 (breast) tumors.