Surfactant is a lipid-rich material which lines the alveolar surface of the lung and which facilitates respiration. The synthesis and or secretion of this material may be affected in a number of disease states. Alveolar type II cells have been demonstrated to be the site of surfactant synthesis but further studies on the metabolism of thismaterial have been slow in forthcoming due to the cellular heterogeneity of the lung and the complicated procedures necessary to obtain isolated populations of type II cells. Recently continuous lines of other differentiated cells have been obtained through transformationwith viruses which are temperature-sensitive (ts) for growth. Such cells are capable of continuous proliferation at the permissive temperature but when incubated at the non-permissive temperature, growth slows and the characteristic phenotype of the cell is expressed. In the present proposal, rabbit type II cells transformed with a ts mutant of the DNA virus SV40 will be similarly characterized for expression of the differentiated phenotype, i.e. characteristic morphology and specific phospholipid and biosynthesis. In addition, cells will be examined for the presence of specific proteins and activities of phospholipid biosynthetic enzymes. These initial studies on development and characterization of a type II cell line which continuously proliferates in vitro yet is capable of expressing differentiated functions would serve as a prelude to further biochemical studies concerned with the metabolism of surfactant and factors which mediate or perturb this process.