Studies will be made of immunosuppression (immunological enhancement) and immunoaugmentation of the immune response to allogeneic and syngeneic tumor grafts in mice. The effects of passive alloantiserum IgG, IgM, F(ab')2, and IgG subfractions on tumor graft growth will be assayed. Assay will be made of the possible immunosuppression of "active enhancement" of tumor allografts by the passive administration of xenoantiserum IgG anti-mouse IgM. The importance of complement (if any) in the suppression in leukemias will be determined with respect to X-ray induced leukemogenesis in two congenic strains of mice, one of which is genetically deficient for C5 and the other is normal for complement.