A titrimetric method for the standardisation of three human urinary trypsin inhibitors (UTIs) has been developed. The same substances are also present in human plasma, and their amounts in such biological fluids are of bioclinical interest. The method is based on the direct titration of each UTI with a solution of bovine trypsin of known molarity, and monitoring the decreasing residual amounts of the inhibitor by reversed-phase high-performance liquid chromatography. The proposed method provides a suitable alternative to the traditional enzymic - spectrophotometric method. Mean within-day and between-day coefficients of variation and the limit of determination for the standardisation of each UTI are reported.