The in vivo immunization of mice with human lymphoblastoid cell line LNH13 generates direct cell-mediated cytotoxicity by spleen cells. The lytic activity appears as early as day 3 after the intraperitoneal inoculation of 7.5 x 10(6) cells and persists at least until day 11. The killer cells do not adhere to plastic and are not retained on nylon wool columns or on Degalan beads coated with mouse Ig plus rabbit-anti-mouse Ig. The effector cells are partly inhibited by treatment with anti-Thy-1.2 serum plus complement, but this inhibition appears to be non-specific since anti-serum alone or normal serum plus complement have the same effects. Heat-aggregated IgG strongly inhibits cytotoxicity, indicating that the effector cells are Fc-positive and that such receptors are implicated in lysis. Altogether, these features strongly argue for an ADCC phenomenon. The involvement of antibodies is demonstrated by the fact that eluates (56 degrees C, 30 min) from immune cells alone induce lysis in the presence of normal spleen cells as effectors. The lytic activity of these eluates can be removed by specific adsorption on protein A coupled to Sepharose beads and on the human lymphoid target cells. Positive complementation between immune and non-immune spleen cells suggest that the arming process may occur in vitro during the assay, when antibodies are released by plasmacytes.