Growth of mouse lymphoma L5178Y, which contains large quantitites of the gangliotriosylceramide (GgOs3Cer), in DBA/2 mice was suppressed by passive immunization with monoclonal immunoglobulin G3 antibodies to GgOS3Cer, but not by immunoglobulin M antibodies with or without added complement. Most groups of mice treated with monoclonal immunoglobulin G3 antibodies did not develop tumors, but the tumor that appeared in a treated animal had a much lower amount of the GgOS3Cer than the cells used for inoculation. Thus, passive immunization either prevented growth of the lymphoma or caused selection of a variant with a lower quantity of the antigen GgOS3Cer.