Recently there has been increasing awareness that deaths of premature infants are frequently associated with the presence of hyaline-like membranes in the lungs. Bruns and Shields1have confirmed the finding of Pichotka and Kühn2in guinea pigs made to breathe high concentrations of oxygen, namely, the development of a hyaline-like membrane in the lungs which bears a striking resemblance to that found in premature infants. These studies have suggested that this histologic picture may be produced by the precipitation of material of endogenous origin, perhaps protein, rather than the inhalation of exogenous material, such as amniotic fluid. The possibility that endogenous protein contributes to the pathogenesis of the pulmonary lesion can be most expeditiously explored by the use of isotopically labele protein fractions. The purpose of the present study, employing I131-tagged protein fractions, γ-globulin and albumin, was to determine the role of these substances, as well