Complete or partial protection against the line-10 hepatocarcinoma was conferred to a significant number of strain-2 guinea pigs by the passive transfer of antibodies reactive to antigens from line-10 cells. Immunoglobulins were administered intradermally and injected sites were subsequently challenged with line-10 cells. Immunoglobulins prepared from the sera of rabbits that had been immunized to line-10 cells and from the sera of guinea pigs that had been rendered immune to this tumor by previously described methods were more effective than immunoglobulins obtained from normal rabbit or guinea pig sera. Protection was abolished if immunoglobulins were absorbed with line-10 cells. Antitumor effects were dependent on the close contact of antibodies and tumor cells. Protection was greater if normal syngeneic peritoneal exudate cells were tranferred in addition to the immunoglobulins. A possible mechanism for the antitumor effects observed is that of antibody-dependent cellular cytotoxicity.