A tumor suppressive composition active against lymphoma consisting of an injection of hybrid protein anti Thy 1.2 monoclonal antibody-ricin and hyperosmotic lactose. The inoculation i.v. of murine tissues in vivo by lymphoma is made at -20 to -25 days and the tumor suppressant composition is used i.v. at Day 1 in an amount of 1-3 .mu.g of anti Thy monoclonal antibody-ricin together with sufficient hyperosmotic lactose to raise the lactose level to 20-30 mM. The broad purpose of this invention is to modify the receptor specificity of a potent toxin such as ricin by coupling it with a monoclonal antibody directed at a specific tumor or differentiation antigen. The object here is to use the reagent to selectively kill tumor cells without affecting normal cells.