Using neutron activation analysis (NAA), the sodium-to-potassium ratio as well as the sodium concentration has been determined for both normal and CF patients. The ratio results correlate with those based on the accepted sodium concentration analysis in the diagnosis of CF and the results also confirm calculations that the sensitivity of the NAA ratio method is a function of neutron energy with the optimum neutron energy being∼1.2 MeV.The coupling of the ratio technique with NAA greatly increases the possibility of performing a successful“sweat test”on an infant and offers the following benefits:Reduction in required sample volume by at least a factor of 3 due to the increased sensitivity of NAAElimination of the necessity to know the amount of sweat, again allowing the use of a smaller sampleElimination of the errors inherent in the present concentration analysis method such as evaporation, electrolytes present from previous perspiration, and variation of electrolyte concentration with sweat rate.