The monitoring of the blood levels of therapeutic antibodies and their immune responses is proposed to guide and optimize therapy with these expensive drugs. We describe a novel Surface Plasmon Resonance (SPR)-based assay suitable for the simultaneous determination of serum concentrations of infliximab and anti-infliximab antibodies. The real-time detection by SPR avoids the incubation/washing steps of commonly used methods, thus allowing faster and more reliable measurements, in particular for low-affinity anti-drug antibodies. This method proved to be highly reproducible and may be well applied to other biotherapeutics.