Vitamin-A is essential for growth and development of cells and tissues. In its active form, retinoic acid, it controls the regular differentiation as a ligand for retinoic acid receptors (RAR, RXR) and is involved in the integration (gap junction formation) of cell formations [Nature 37 (1994) 528; International Review of Cytology. San Diego Academic Press, 1-31]. Vitamin-A plays a substantial role, especially in the respiratory epithelium and the lung. During moderate vitamin-A-deficiency, the incidence for diseases of the respiratory tract is considerably increased and repeated respiratory infections can be influenced therapeutically by a moderate vitamin-A-supplementation [Aust. Paediatr. J. 22 (1986) 95; Lancet 338 (1991) 67]. In addition to the importance of the vitamin for the lung function, vitamin-A is also responsible for the development of many tissues and cells as well as for the embryonic lung development. Recent studies proved that the control occurs by different expressions of retinoid receptors as well as by time-dependent changes of the vitamin-A-metabolism respectively via cellular vitamin-A-binding proteins (CRBP: cytoplasmatic retinol binding protein; CRABP: cytoplasmatic retinoic acid binding protein).