The activation of EL-4 T-cells is investigated when the T-cells are sensitized with monoclonal antibodies against CD3/CD28 followed by irradiation with atmospheric oxygen plasma. The proliferation of anti-CD3/CD28-sensitized EL-4 T-cells was evaluated as a function of the oxygen plasma irradiation duration. The plasma irradiation promotes cell proliferation: when plasma irradiation was applied to sensitize T-cells for 40 s, the cell number increased by five times over 24 h compared with that in a cell population that was not exposed to irradiation. Also, the plasma irradiation increases the production of interferon gamma in T-cells by a factor of two. Gene expression analysis of T-cell supports results of the proliferation and the immune function enhancements. (C) 2020 The Japan Society of Applied Physics