The objective of our research is to elucidate the molecular mechanisms that regulate the proliferation and differentiation of tracheobronchial epithelial cells. A clear insight into the basic mechanisms of these processes is required in order to understand the mechanisms involved in tumor development and other pathological conditions. To study the regulation of differentiation of the tracheobronchial epithelium, an in vitro cell culture model was developed using rabbit tracheal epithelial cells. Our studies were directed towards answering the following questions: (1) What is the differentiation capacity of these cells? (2) Does differentiation occur as a multistep process? (3) What factors control differentiation and by what mechanism? (4) What are the biochemical markers for the various differentiated phenotypes and at what level are they controlled? (5) Is there a link between control of cell growth and control of cell differentiation?