A sensitive microassay for protein in cells cultured on collagen

A microassay for protein that is linear from 0.1 to 5 μg of protein and does not detect collagen has been developed. The assay is based on the ability of bromosulphophthalein (BSP) to form BSP-protein complexes which precipitate at low pH. Maximum precipitation occurs when 30 or more BSP molecules are bound per albumin molecule. Collagen is not detected because too little BSP binds to this protein to precipitate it. This assay should be of great value to those who grow dispersed cell cultures on a collagen substrate.