The present application aims at providing a treatment method effective for various cancers, immune deficiency diseases and infections, and a method of proliferating/activating cells associated therewith, which can be performed at such low costs that the methods can be applicable to nonhuman animals.A method of proliferating/activating cells of the present application comprises steps of, during the cell culture, supplementing a plant lectin (such as concanavalin A) and a growth factor having interleukin-2-like activity to the culture medium. Accordingly, the present method can proliferate/activate, on a preferential basis, aβ-type T cells associated with cell-mediated immunity for cancers, immunodeficiency diseases and infectious diseases.