Amperometric titration of various protein fractions and human serum with 12-phosphotungstic acid at the rotating gold electrode is described. The titrant is electroreducible and its concentration may be monitored by poising an amperometric indicator electrode at a potential of -0.25 V vs. SCE. In 1.0 M hydrochloric acid, well defined titration curves of proteins may be obtained in the presence of airsaturated solutions. The relative standard deviation of replicate samples is 0.2-0.3% in the range 2-4 g of protein per 1 in the sample solution. The technique is principally applicable to homogeneous mixtures of proteins.