Cryostat sections of methylcholanthrene-induced murine tumors were previously shown to adsorb erythocytes sensitized by subagglutinating concentrations of antierythocyte antibodies. This type of hemadsorption was seen also with sections of normal murine spleen. It was shown in the present study that immune complexes composed of bovine serum albumin (BSA) and rabbit anti-BSA serum, or human IgG and mouse anti-human IgG serum inhibit adsorption of sensitized erythocytes by tumor and spleen sections. Maximum inhibition was produced by complexes formed at excess of antigen. In contrast inhibition was not seen with complexes formed at equivalence. In addition, heat-aggregated human IgG, but not monomeric human IgG, inhibited hemadsorption by tumor tissue. Anti-BSA, antihuman IgG, BSA or human IgG alone did not produce significant inhibition. These results support the contention that murine tumor cell and lymphocytes have similar Fc receptors on their surface. The implication that the nonspecific attachment of immune complexes to the tumor cells and lymphocytes may alter the immunological response of the host to the tumor is discussed.