Determination of the carbon concentration in protein solutions by total organic carbon analysis was found to be a sensitive and reliable method for the estimation of protein concentrations. Using a carbon content of 0.53 g/g in protein and of 0.44 g/g in carbohydrate, the concentrations of normal proteins, proteins containing chromophoric groups, and proteins containing carbohydrate could be established. The method appeared to be independent of the nature of the protein and showed complete linearity between 25 and 1000 mg/l (0.5-20 micrograms per assay) when protein was serially diluted. Determination of specific absorption coefficients by measuring both the absorbance of protein solutions at 280 nm and their carbon concentrations gave values which, on the average, coincided within 12% with values reported in the literature. The method may have special applicability in protein purification studies, as it does not require knowledge of molar extinction coefficients beforehand, and also monitors the disappearance of carbon compounds other than protein.