The standard Bradford protein assay is insensitive to collagen. But if a small, sub-threshold amount of SDS is added to the sample, the response to collagen in increased by at least an order of magnitude, while, on average, the sensitivity for non-collagens is decreased by approximately a factor of 2. As a result comparable color formation is achieved with both collagens and non-collagens. The addition of protein to a sub-threshold amount of SDS results in the formation of a green color measurable as an increase in absorbance at 700 nm, in contrast to the blue color measured at 595 nm in the standard assay. Depending upon the source, the threshold level for SDS varies from 30 to 50 μg. The response to protein is linear up to approximately 40 μg of protein per ml of reagent. © 1981.