Cystic fibrosis (CF) is a severe autosomal recessive disorder. It is caused by mutations in the CF transmembrane conductance regulator gene. Early diagnosis of CF can be carried out by determining high immunoreactive trypsinogen (IRT) blood values in newborns. A simple sandwich-type ultramicroELISA assay (UMELISA (R)) has been developed for the measurement of IRT in dried blood spots on filter paper. Strips coated with a high affinity monoclonal antibody directed against IRT are used as solid phase, to ensure the specificity of the assay. The assay is carried out within 20h. The useful rank of the curve is 0-500ng/mL, and the lowest detectable concentration is 4.8ng/mL. Intra- and inter-assay coefficients of variation were lower than 10%. The recovery mean value was 100.3 +/- 11.2%. Cross-reactivity with proteins structurally related to IRT (2-macroglobulin, 1-antitrypsin, and human chymotrypsin) was lower than the detection limit of the assay. Four thousand four hundred six newborn samples from the Cuban Newborn Screening Program were analyzed, and the mean IRT concentration was 12.8ng/mL. Higher IRT values were obtained when samples were eluted overnight. Regression analysis showed a good correlation with the commercially available AutoDELFIA (R) Neonatal IRT kit (n=3948, r=0.885, ?=0.976, p<0.01). The analytical performance characteristics of our UMELISA (R) TIR Neonatal suggest that it can be used for the neonatal screening of CF.