The cytotoxic activity of healthy donor lymphocytes toward cells of two human lymphoblastoid lines was studied by radioimmunoassay with 51Cr. Cells of the following lymphoblastoid lines were used as target cells: line L-101, derived from the peripheral blood of a patient with acute erythromyelosis, and line D-102, derived from the blood of a healthy donor. Lymphocytes of different donors revealed a certain cytotoxicity toward these target cells (from 0 to 34.4% for cells of L-101 line and from 3.1 to 37.2% for cells of D-102 line). Attempts to enhance the mentioned cytotoxic effect with interferon were made. Human leukocyte interferon was obtained by induction with Newcastle disease virus. The preparations of interferon were concentrated 10 to 20 times by ultrafiltration. As a control, preparations of “false interferon” were used, having been obtained and concentrated in the way analogous to interferon obtaining, except for viral induction. Preincubation of donor lymphocytes in interferon of different dilutions during 1 to 2 hr under 37°C led to enhancing their cytotoxic action toward lymphoblast target cells 1.5 to 30.6% higher in comparison with preincubation in the medium RPMI 1640 or in “false interferon.” A certain enhancing effect of “false interferon” preparations on the cytotoxicity of normal human lymphocytes toward lymphoblastoid target cells was observed, but in these cases the cytotoxicity value was significantly lower than that under the treatment with interferon. The probable role of interferon in immune reactions between lymphocytes and target cells is discussed.