The development of a simple and reliable technique of trypsin radioimmunoassay in blood eluates allowed the neonatal screening for cystic fibrosis in a province of France. The study of the distribution of blood immunoreactive trypsin levels in 79,880 five day-old neonates led to choose a cut-off value of 900 micrograms/l; this choice gives a false positive rate of 3 in 1,000. The evolution of trypsin levels according to age during the first weeks of life leads to select a "two-phase" screening strategy: only neonates whose high neonatal trypsin levels persist after the second week of life (4 in 10,000) will undergo sweat test. Twenty-three cases of cystic fibrosis were diagnosed in this study, which gives an incidence of 1 in 3,470.