The biotin/avidin system is one of the most widely used affinity detection and affinity capture systems in biology. However, the determination of the exact number of biotin tags attached onto a substrate is complicated by the fact that biotin does not present any light-absorbing or -emitting properties. Here, we describe a fluorescent biotinylation reagent designed from the general multifunctional single-attachment-point (MSAP) reagent concept. A Lys-Cys dipeptide scaffold was used to display a biotin functional group and a fluorescein functional group along with an N-hydroxysuccinimide ester reactive group. The resulting bifunctional MSAP reagent, Fl-Biotin-NHS, was used to prepare a monobiotinylated version of cetuximab, which was further reacted with avidin to obtain a soluble avidin-based cetuximab oligomer. The MSAP peptide-scaffold approach allows fluorophores, chromophores, or reactive groups to be combined with biotin and provides a broad approach to obtain multifunctional biotin-based reagents.