Differences can be demonstrated between the lysis of xenogeneic target cells by lymphocytes of injected rats, which is inhibited by antiglobulin serum, and the lysis of antibody-coated xenogeneic target cells by normal rat lymphoid cells, which is much less readily inhibited. This difference is demonstrated by the dilution of antiglobulin which is effective, and by the fact that inhibition of lysis produced by allergic lymphocytes can be shown if the lymphocytes are treated with antiglobulin and then washed. Differences can also be shown in the time course of inhibition, since to be effective antiglobulin must be present at the time of mixing normal lymphocytes and antibody-coated target cells, whereas addition of antiglobulin to allergized lymphocytes 4h after mixing with target cells still produces significant inhibition. These results are compatible with the hypothesis that allergic lymphocytes have immunoglobulin on their surfaces and kill by a different mechanism from the lymphoid cells which lyse antibody-coated target cells.