Mast cells taken 11 or 25 days after a sublethal irradiation (540 R) formed rosettes with antibody-coated sheep erythrocytes in the same number as in control non-irradiated mice. The threshold of passive cutaneous anaphylaxis reactions with mouse IgG1 or IgE antibody 2 to 7 days after irradiation was the same in sublethally (540 R), or lethally (780 R), irradiated mice as it was in controls.