A method is described for the sensitive measurement of adsorbed proteins using femtoliter microwells. Quantitative measurement of adsorbed protein is demonstrated at surface densities from 10 fg/cm(2) to 3 pg/cm(2). Determination of the efficacy of barrier coatings is also demonstrated using femtoliter microwells. Adsorption at low surface densities is measured, indicating the highest affinity sites on the surface and therefore the initial stages of adsorption. The femtoliter microwell method is shown to be useful in detecting differences between effective protective coatings.