Membrane lipid bilayer fluidity has a crucial role in signal transduction for a variety of biologically active molecules which activate cellular functions, cell differentiation and proliferation. No general trend is discernible in tumors regarding the direction of alteration in membrane fluidity. In hepatomas, the membrane fluidity decreases, whereas in neural tumors, lymphomas and leukemias, the membrane fluidity seems increased. The capability of cancer cells to metastasize seems to be related to the degree of membrane lipid fluidity. The possibility that cancer cell plasma membrane contains cryptic antidromic antigens becoming exposed by altering the fluidity, has triggered a potentially new approach in the treatment of neoplastic diseases. Further exploration into the mechanisms by which biologically active substances alter the membrane fluidity, may provide clues for understanding the mechanism of tumor growth and cancer metastases.