A method of quantitating proteins in complex samples using capillary electrophoresis can be used both to determine the concentration of a protein in a sample and to determine total protein concentration in the sample. In general, the method of determining the concentration of a marker protein comprises: (1) adding a known quantity of an internal standard compound to a sample containing at least one protein, the internal standard compound selected from the group consisting of benzoic acid substituted with at least one halogen, producing a detector signal in relation to its concentration, and being capable of electrophoretic separation from the protein; (2) subjecting the sample and the internal standard compound to capillary electrophoresis to separate the protein and the internal standard compound from each other and from other components in the sample; (3) measuring the detector signal produced by the internal standard compound and a detector signal produced by the protein to determine a ratio of protein signal to internal standard signal; and (4) determining the concentration of the protein in the sample from a standard curve of protein concentration versus the ratio of protein signal to internal standard signal. A typical protein measurable by the method is albumin. Typically, measurements are made at 214 nm and the internal standard compound is 2,4-dichlorobenzoic acid.