Growth factors are polypeptides, which trigger a biological action in the cell by modulating cell proliferation and differentiation. They are hormone-like substances and include various families such as epidermal and platelet-derived growth factors, neurophins, insulin-like, transforming, heparin-binding, hepatocyte, vascular endothelial and hemopioetic growth factors. These factors are recognized by specific receptors, which are high molecular weight and membrane-associated glycoproteins. Most of these receptors exhibit protein-tyrosine kinase (PTK) activity. This PTK activity is stimulated by binding of the receptor to its specific factor as a ligand. The PTK that catalyzes this phosphorylation is associated with the growth factor receptor itself (autophosphorylation). Phosphorylation of other cellular proteins may occur by growth factor receptors, which possess this activity. Also, PTK regulates cell proliferation, differentiation and immune processes.  Moreover, PTK activity is involved in cell signaling through phosphoinositide turnover. Uncontrolled signaling from receptor and intra-cellular tyrosine kinases can lead to numerous proliferative diseases. Accordingly, some enzymes are regulated by growth factor receptors-associated PTK. These reported enzymes include phospho-lipases C and D, phosphoinositide-3-OH kinase, 17 ß-hydroxysteroid dehydrogenase, GTP-utilizing enzymes, adenylate cyclase, glycolipid sulfotransferase, protein tyrosine phosphatase 1B, phospholipase A₂ and mitogen-activated protein kinase. The involvement of PTK in the regulation mechanism of some enzymes is confirmed by using some inhibitors such as tyrphostins and genistein as specific inhibitors, which block its activity. Tyrphostins are considered as signal transduction agens. In addition to their effect on PTK, these inhibitors exert some actions on the cellular events.