Costimulation of SeAx cells by nonmalignant cells and SEA is blocked by an anti–IL-2 mAb. Malignant T cells (SeAx) were grown with or without irradiated (to avoid proliferation) nonmalignant CD4⁺ T cells (P1675), and/or SEA (100 ng/mL) with or without control mAb or an IL-2–blocking (25 μg/mL) mAb in microtiter plates. Twelve hours prior to harvest, ³H-thymidine (1 μCi [0.037 MBq]/well) was added, the cells were harvested onto glass fiber filters, and ³H-thymidine incorporation was measured. The proliferation was expressed as mean counts per minute of triplicate cultures. Essentially identical results were obtained in 2 independent experiments. Error bars represent SD.