The fixation of antibodies directed against viral induced antigens was observed on SV40 transformed hamster fibroblasts. These cells were incubated for various periods of time with sera of tumor bearing hamster in the presence or absence of fresh guinea pig complement. The fixation of antibodies was revealed by an anti hamster immunoglobulins immune serum labeled with 125 iodine. Autoradiographic observations were made by optic and electron microscopy. The fixation of antibodies is quite different in the presence or in the absence of complement. Cells incubated with antibodies alone show a regular label on the plasma membrane with no decrease in the total cell population as compared to the controls. When the complement is added, one fraction of the cells is heavily labeled detached and progressively destroyed. Some of the cells (2/10) still attached on the flask are heavily labeled on the plasma membrane, the major part being nearly negative.