By injection of gelatin-India ink and latex masses under controlled conditions into the lungs of infants with asphyxia associated with hyaline membrane formation, it has been shown that such membranes prevent the filling of the distal alveoli. It is postulated that these membranes obstruct the flow of air to the distal alveoli and prevent their expansion, thus decreasing the compliance of the lung. Plasmin, introduced into the lung, has been shown to cause dissolution of the hyaline membranes in vitro.