A thermometric titration for the determination of total serum proteins is reported. The titrant, 12-phosphotungstic acid, reacts stoichiometrically with the protonated basic amino acid residues of a protein. High purity cesium chloride may be used for standardization of the titrant. Both reactions are carried out in 0.1 M hydrochloric acid. Results indicate that either a pure protein (bovine serum albumin) or mixture of proteins can be titrated in solutions as dilute as 1 g/l., which is equivalent to a 1.5mM solution of the reactive group, or as concentrated as 15 g/l. Under appropriate conditions, including the rate of addition and stirring, the stoichiometry is independent of protein concentrations and the end point is precise to 0.5%. Interferences from nitrogen bases such as bilirubin at the levels commonly encountered in serum appear to be negligible. At present, the protein content of 1 ml of human serum can be determined with a precision of 0.5%. Correlation of the method with Kjeldahl nitrogen analysis and biuret colorimetry is acceptable.