The concomitant appearance of enterokinase (EK) and trypsin activities in the human intestinal mucosa is indicative of the importance of EK as an activator of trypsinogen and therefore as the key enzyme in protein digestion. Enterokinase can be detected in fetal mucosa from the 26th week of gestation on, paralleling appearance of tryptic activity in meconium. The developmental pattern of EK activity increases with age. Between 26 to 30 weeks of gestation, the EK activity is only 6% and full term babies (40 weeks) 20% of that found in older children. In contrast, lactase studies during development show a lactase activity of only 30% in human fetuses between 26 to 34 weeks of gestation as compared to full term babies. During the same gestational period, sucrase and maltase activities reach 70% of the full term. In addition, the distributional pattern of EK differs from the disaccharidases, showing the highest activity in duodenum and the lowest in ileum, whereas disaccharidases are highest in jejunum with lower activity in duodenum and ileum. Differences in topographical distribution and time of appearance of EK and disaccharidases may be attributed to differences in orgin as well as subcellular localization of these enzymes. It is conceivable that the premature infant, between 26 to 30 weeks of gestation, is better equipped to deal with hydrolysis of alpha-glucosides than of lactose.