We have used surface plasmon oscillations to monitor the time evolution of the adsorption of antibodies from aqueous solution onto an antigen-coated surface. The asymptotic adsorbance values (Gamma(f)) do not reach a maximum until the concentration of antibody in solution (C) is greater than 1 mg/mL. We interpret the shape of the Gamma(f) vs C plot as an evolution from side-on adsorption to end-on adsorption. Our data show a characteristic time, t(c), which is proportional to C-2, indicating diffusion-limited adsorption. The value of t(c) is significantly greater than predicted from a diffusion-limited model. We attribute this result to a requirement that the antibodies be correctly oriented if they are to be adsorbed. A careful study of our kinetics data shows that for high concentrations we must apply theories of random sequential adsorption in addition, because the adsorbed molecules may exclude more surface area than they physically occupy.