The concentration of specific alloantibody in purified mouse immunoglobulin preparations was determined. When passively transferred in adequate doses, IgM, IgG1, and IgG2 antibodies all induced tumor enhancement in allogeneic hosts. IgM and IgG2 antibodies in high concentration led to inhibition of tumor growth. IgM and either IgG1 or IgG2 had additive effects on tumor enhancement. IgG1, but not IgG2, suppressed the inhibitory effect of IgM in high concentration.

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