Abstract Determination of factor XII was performed in a test system where plasma samples were diluted (1/400-1/3200) in buffer and factor XII deficient plasma. Diluted Cephotest R was used as activator and after 10 min activation time chromogenic substrate H-D-Pro-Phe-Arg-pNA was added. The assay was adopted to an enzyme analyzer. The reproducibility of the method was acceptable with a coefficient of variation of 7.8% in the normal range. The correlation of this method to a one-stage clotting assay for factor XII was good, r 0.79. The preliminary reference interval was calculated to be 52–147% of normal (mean+ 2 S.D.).