Comparative measurements on the levels of immunoreactive trypsin(ogen)-1 and lipase have been performed on newborn and adult sera. Values observed for immunoreactive trypsin(ogen)-1 with newborn sera are more dispersed and significantly higher (mean +/- SD, 31.9 +/- 12.9 micrograms/l) than the values obtained with adult sera (20.2 +/- 7.3 micrograms/l). The molecular size distribution of the immunoreactive trypsin-like material in newborns was studied by gel filtration. This material was eluted with a molecular weight of 25,000 which eliminates the possibility of an increased intestinal permeability of active trypsin which would be bound to serum protease inhibitors. In contrast to trypsin values, the mean serum lipase value at birth was significantly lower (7.0 +/- 3.2 micrograms/l) than the value observed for adults (26.9 +/- 8.2 micrograms/l). The possibility of an adaptation process in newborns is evoked.