Surface receptor sites for the Fc portion of antigen antibody complexes were demonstrated on cells derived from three methylcholanthrene induced fibrosarcomas, one of strain C3H and two of strain BALB/c origin, two spontaneously occurring malignant melanomas (B16 in strain C57BL/6 and Harding Passey in strain BALB/c mice), a Moloney sarcoma virus induced tumor of strain BALB/c origin and the Walker 256 carcinosarcoma of Holtzman rats. Primary cell cultures derived from these tumors adsorbed technetium 99m labeled, antibody sensitized sheep erythrocytes (99mTc EA) as determined either by visual scoring of adherence or radioisotopic quantitation. Depending upon the tumor tested, from 20% to more than 95% of the target cells adsorbed 99mTc EA. All cells lost their reactivity after 1 or 2 passages in vitro, but this was regained after a single passage in vivo. Indicator erythrocytes coated with F(ab')2 fragments of the sensitizing sheep erythrocytes (SRBC) antiserum did not adhere thereby demonstrating that the hemadsorption required an intact Fc portion of the antibody molecule. Adherence of 99mTc EA was blocked by soluble immune complexes prepared with ovalbumin and rabbit antibody directed against it and Escherichia coli 055:B5 lipopolysaccharide and mouse antibody directed against it. Normal rabbit or mouse serum, immune serum, or antigen alone did not block adherence of 99mTc EA thereby demonstrating that the receptors had greater affinity for immune complexes than for either antigen or antibody alone. The existence of membrane receptors on tumor derived cells which react with the Fc portion of antigen antibody complexes may provide an explanation for the mechanism by which immune complexes are capable of blocking cell mediated tumor cell destruction irrespective of whether the receptors are on the tumor cells themselves or on admixed lymphocytes and macrophages.