Peripheral blood lymphocytes (PBLs) from 28 patients with non-Hodgkin’s lymphoma (NHL) were compared with those of 28 healthy subjects (controls) for their responsiveness to the T cell mitogen phytohemagglutinin (PHA) before and after treatment with γ-radiation and mitomycin C. PBLs from 9 of the 28 patients with NHL exhibited almost a total failure to respond to PHA while mitogenic stimulation in the rest varied from 7 to 90% relative to the controls. PBLs from 19 NHL patients and 28 healthy subjects were compared for their radiosensitivity by measuring postirradiation uptake of [3H]-thymidine. Increased sensitivity to γ-radiation was observed in more than half of the NHL patients studied. Increased sensitivity to mitomycin C was also noted in 5 of the 7 patients analyzed compared to the respective controls. Poor mitogenic responsiveness and hypersensitivity of blood lymphocytes to the carcinogens (γ-rays and mitomycin C) are suggestive of a severe immunological abnormality and defective DNA repair in these Saudi NHL patients.