Growth factors are polypeptides which exert their mitogenic action through binding to specific high-affinity receptor molecules on the cell surface of target cells. This interaction leads to the rapid activation of a receptor-linked signal transduction system, involving the stimulation of an intrinsic receptor tyrosine phosphokinase activity, the breakdown of inositol lipids, and the production of ionic signals. In this contribution we have analysed the nature and origin of the ionic signals, and we have applied monoclonal antibodies against the receptor for epidermal growth factor (EGF), as well as tumour-promoting phorbol esters, to dissociate the early cellular responses to growth factors. Evidence is presented that the ionic signals are coupled to the breakdown of inositol lipids. The hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) would lead to the production of 1,2-diacylglycerol (DG) and inositol triphosphate (IP3). DG production results in the stimulation of protein kinase C, which causes the activation of Na+/H+ exchange by increasing its affinity for cytoplasmic H+. Consequently, a rise in cytoplasmic pH is observed. This response can be mimicked by the tumour promoter TPA, which can replace DG in activating the protein kinase C. Independently, IP3 production leads to the rapid mobilization of Ca2+ from intracellular stores. Monoclonal antibodies against the EGF receptor differed in their ability to evoke EGF-like responses upon binding to the EGF receptor. Of the three anti-EGF receptor IgGs tested, one was directed against the EGF-binding domain (2E9), and the others (2D11 and 2G5) were directed against sugar moieties not involved in EGF binding. Receptor tyrosine kinase activity could be stimulated by 2E9 as well as 2D11, but not by 2G5. Only 2D11 induced morphological changes similar to EGF. None of the antibodies was able to trigger the production of ionic signals, which implies probably that antibody binding to the receptor, even when they bind to the EGF binding domain, is an insufficient stimulus for the breakdown of inositol lipids. Most importantly, these monoclonal antibodies were also not able to induce DNA synthesis in quiescent human fibroblasts, not even after cross-linking of the EGF receptors by a second antibody. It may thus be concluded that the stimulation of the intrinsic receptor tyrosine phosphokinase activity can be dissociated from other early responses, and that none of the identified early responses is a sufficient trigger for the mitogenic action of growth factors.