Culture conditions were examined for colony formation of human peripheral blood lymphocytes in vitro. It was found that addition of 1% human serum to the medium together with fetal calf serum greatly improved the cloning efficiency and colony size, moreover allogeneic lymphocytes and lymphoblastoid cells are both required as feeder cells for better results. The X-ray dose-survival study showed that the radiosensitivity of lymphocytes remained essentially the same whether the irradiation was performed prior to separation of the lymphocytes from blood or 4 h after addition of phytohemagglutinin to the separated lymphocyte culture; however, the sensitivity was definitely increased as the cell cycle progressed from G0 to G1/S or log phase.