Effect of Inhibitors from Adult Connective Tissue on Growth of a Series of Human Tumors in Vitro.

factors (growth stimulating and growth inhibiting sub stances) present in normal adult tissue form a growth control mechanism which is responsible for the dormancy of normal adult tissue. An upset of this balance, or the absence of the inhibiting factor, may result in the un restrained growth of cancer. The thesis that neoplasia may represent a disturbance of equilibrium between growth stimulating and growth inhibiting substances was proposed by Murphy (7). Paul (14) in a recent review of the cancer cell in vitro, stated that “Cancer is clearly the result of a defect of the homeostatic mechanisms which maintain the balance among cells in the organism.― The isolation and characterization of specific growth factors from normal adult mice by Levi-Montalcini and Cohen (2, 6), as well as the extraction from normal thymus and other tissues of “promine― and “retine― by Szent-Gyorgyi et at. (5, 19—21) has renewed interest in this hypothesis. SiImns and Stillman (17, 18) described the removal of a “tissueinhibitor― present in adult tissue and in tissue cultures of adult tissue, by enzymatic digestion. Later Simms and Parshley (16) observed that either growth stimulators or inhibitors were produced by enzymatic