Present concepts of the roles of collagen and elastin in lung elastic behavior and maintenance of lung structure have been largely inferred from anatomical observations or from studies of isolated fibers in vitro. Based on the intimate association of elastin and collagen it has been postulated that elastin contributes little to elastic behavior and that collagen is the major determinant of lung structure. Using clostridial collagenase, pancreatic elastase, and papain we have selectively degraded these fibers and studied the resulting changes in elastic behavior and structure of rat lungs in vitro.Pressure-volume curves were recorded during continuous slow air inflation and deflation (10.5 ml/min) before and after the intratracheal instillation of 0.5 ml of control or enzyme solution. Surface tension-lowering activity of lavaged material was studied. All lungs were fixed inflated at 25 cm H(2)O pressure and whole lung sections were stained for elastin, collagen, and reticulin. Collagenase produced a marked susceptibility to pleural rupture but did not alter elastic behavior or lung structure. Elastase and papain produced segments of lung with increased compliance; this change was not due to alteration in surface forces but was associated with decreased tissue elastic recoil. Histologically, altered tissue recoil correlated well with evidence of damaged elastin fibers. In contrast to previous concepts these results suggest that elastin is the major connective tissue determinant of lung structure and elastic behavior.