In 70 newborns with respiratory distress syndrome (RDS) and in roughly the same number of eutrophic mature newborns the total antiplasmin, progressive antithrombin and alpha 2-macroglobulin was determined, the latter in an enzymatical as well as immunochemical way. In healthy mature newborns the progressive antithrombin was somewhat below the level of adults, alpha 2-macroglobulin was above it in both methods and total antiplasmin within it. All parameters of protease inhibitory capacity were significantly lowered in newborns with RDS. Smaller values could be found in patients with bleedings and in those who died later on. Even in those patients with a birth weight under 2,000 g there was a tendency to lower values which can only be partially due to the specificity of development of inhibitors. Progressive antithrombin, total antiplasmin and alpha 2-macroglobulin determined enzymatically are correlated in newborns with RDS jointly and with numerous other parameters of the coagulation system. These relations point to the fact that all components are included in the same consumption process, viz. in the process of disseminated intravascular coagulation. Alpha 2-macroglobulin values determined immunochemically do not correlate with coagulation parameters determined enzymatically as well as with other parameters. They lay partially above or below those determined enzymatically. This behaviour can only be explained by a partial enzyme complex binding of alpha 2-macroglobulin in newborns with RDS.