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Two immunotoxins containing ricin A-chain, staphylococcal protein A and mouse anti-Thy 1.2 antibodies of IgG(2a,b) subclass, were prepared. The two multivalent immunotoxins of 750 and 370 kDa and molar ratio A-chain: IgG of 1:2, were used for the treatment of mice bearing ascitic EL4 lymphoma. The immunotoxin-treatment, performed intraperitoneally, was combined or not with homologous or heterologous interleukin 2. The antitumor effects expressed by increase of mice survival time (p < 0.001 as compared with nontreated animals) corresponded to a proportion of 88-90% lymphoma cell-kill by immunotoxin and 93-95% by combination treatment (immunotoxin + interleukin 2), as calculated from the relationship between the survival time of nontreated mice and the number of tumor cells inoculated.