Measurement of total protein is essential to monitor the progress of the purification of a desired protein. Total protein is typically measured in the supernatant following extraction and clarification by centrifugation. Various methods are employed for the estimation of protein, but none is free from shortcomings. A major disadvantage for most protein assays is the significant amount of variation among proteins (Table 2.1). Error is even greater if the protein contains non-proteinaceous groups such as carbohydrates. Dry weight determination gives a value for the whole molecule, but is not advantageous when one is working with sub-milligram-size samples.