Animal models have demonstrated that syngeneic lymphocytes activated ex vivo and infused into animals with experimentally induced tumors can mediate tumor regression. This "adoptive immunotherapy" has been applied to patients with end-stage malignancy refractory to standard therapy. Lymphocytes are collected with the blood cell separator, expanded in culture under the influence of cytokines such as interleukin-2 (IL-2), and reinfused into the patient under conditions similar to those used in the animal models. Studies from several centers using lymphokine-activated killer (LAK) cells, involving more than 300 patients, have shown an overall response rate of greater than 15% and a complete response rate of approximately 10%. Renal cell carcinoma, melanoma, and lymphoma appear to be the cell types that respond best to such therapy. Toxicity in these phase 1 studies has been substantial, related primarily to high doses of intravenous IL-2, and treatment-related deaths have been reported. Adoptive immunotherapy using lymphocytes derived from surgically excised tumors, tumor-infiltrating lymphocytes (TIL), is in the early stages of clinical trials, but this appears to offer a potentially more potent and specific approach than does LAK cell therapy. TIL have been shown to traffic to tumor sites and mediate tumor regression. The mechanisms of adoptive immunotherapy are poorly understood, but blood cell separators and storage technology are playing a critical role in the collection and processing of cells for these research applications.