Abstract: A new immunotherapy to cancer patients is provided. In the therapy, lymphocytes collected from a patient by leukopheresis are treated with interferon-.alpha. in vitro for a short period of term and then returned to the patient intravenously. According to the therapy, reduction in sensitivity of tumor cells to natural killer-cell lysis due to administration of interferon-.alpha. is eliminated, the activities of NK cell and 2-5AS and interferon-producing capacity can be enhanced, a large amount of lymphocytes can be activated directly in vitro with the dose of optimally adjusted to the patient, none of side effects are found, and any other potentiators and comodulaators, which may not be allowed to use in vivo, can be added to the lymphocytes and removed therefrom before returning to the patient.