Lungs of adult rats were lavaged with normal saline containing 0.25 mM phenylmethylsulfonyl fluoride. The surfactant pellet was removed by centrifugation and serum proteins in the lavage were removed by affinity chromatography using rabbit anti-rat whole serum antiserum. The residual proteins, thought to represent products of secretory cells, were used as the immunogen to inject rabbits. The resulting antiserum was absorbed with affinity columns of rat serum and rat liver extract. The gamma globulin fraction of the unbound antiserum was found to react with two proteins in the lavage by immunodiffusion and crossed immunoelectrophoresis. The antiserum specifically stained, by the immunoperoxidase method, a subpopulation of cells consistent in morphology with Clara cells lining the bronchioles and bronchi. The antigens were detectable, by immunohistochemistry, in rat fetus at 19 days of gestation, a progressive increase in the antigen content was noted with increasing gestational age and an adult pattern was noted at 2 weeks of age. In adult animals the intracellular content of the antigens appears to be about twofold greater than their content in the lavage fluid.
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