The clinical value of plasma trypsin determination by radioimmunoassay has been investigated in children. In newborns (1-7 days), plasma trypsin levels are significantly higher than in older children, whose values do not differ from the adults' age. Variation between 15 days and 18 years is negligible. In ten cases of mumps without pancreatic symptomatology, results are within the normal range, even when amylase levels are high. In cystic fibrosis plasma trypsin has been found undetectable or very low in 13, normal in 2 and high in 1 of 15 cases. Plasma trypsin levels have also been found undetectable in 18 of 29 diabetic children, some of whom had been diagnosed for less than a year. We conclude that plasma trypsin determination by radioimmunoassay is of interest in pancreatic function testing of children.