A population of tumor-infiltrating lymphocytes (TILs) harvested from a patient and transduced with a retroviral vector encoding the gene for interleukin-2 (IL-2). TILs require IL-2 to sustain them when they are transferred back to the patient. By inserting the IL-2 gene into TILs in vitro so that TILs express IL-2 continuously it is possible to provide TIL-sustaining IL-2 in vivo specifically at the site of lymphocyte infiltration, thereby avoiding the toxicity associated with the systemic administration of IL-2. (NCI04) [ ]