The growth of normal cells is largely controlled by the interplay between several polypeptide hormones and hormone-like growth factors that are present in tissue fluids.¹ Many new polypeptide growth factors have recently been identified in blood, serum, tissue fluids, and cellular extracts.^{2 ,23} Malignant cells, however, are not subject to all the same growth controls as are normal cells. In general, malignant cells require less of these exogenous growth factors than do their normal counterparts for optimal growth and multiplication, and it has been suggested that "transformed or malignant cells escape from normal growth controls by requiring less of [such] hormones . . .