After specific stimulation by the appropriate growth factors, tyrosine kinases phosphorylate themselves on tyrosine as the first step in the cascade of events ending in the promotion of deoxyribonucleic acid (DNA) synthesis and cell growth. To idenfity new growth factor receptors, antiphosphotyrosine antibodies have been produced and used to screen a variety of normal and tumour cell model systems. In 2 instances antiphosphotyrosine antibodies identified a new tyrosine kinase. One (p190met) turned out to be a growth factor receptor expressed by epithelial cells and tumours of the gastrointestinal tract. The other (p56lck) was found to a transmembrane signal transducer coupled to a lymphocyte receptor, involved in the immune response and deregulated in leukaemia cells. In response to their specific extracellular ligands, these cell membrane molecules activated their tyrosine kinase activity and transduced the mitogenic signal into the cell's internal machinery. Under physiological conditions, the process was tightly controlled by the exogenous ligand. Pathological constitutive autoactivation of the tyrosine kinase was observed only in the case of structural alterations of the receptor itself or in autocrine loops, where the same cell simultaneously expresses both the growth factor and the receptor.