- This work evaluates the concept of a partial-filling technique in affinity capillary electrophoresis (ACE) using two model systems: vancomycin from Streptomyces orientalis and carbonic anhydrase B (CAB, EC 184.108.40.206). In this technique the capillary is first partially-filled with ligand followed by a sample of receptor and non-interacting standard and electrophoresed. Analysis of the change in the mobility ratio, M, of the receptor, relative to the non-interacting standard, as a function of the concentration of the ligand, yields a value for the binding constant. These values agree well with those estimated using other binding and ACE techniques. Data demonstrating the quantitative potential of this method is presented.