We used atomic force microscopy to measure the binding forces between Mucin1 (MUC1) peptide and a single-chain variable fragment (scFv) antibody selected from a scFv library screened against MUC1. This binding interaction is central to the design of molecules used for targeted delivery of radioimmunotherapeutic agents for prostate and breast cancer treatment. Our experiments separated the specific binding interaction from nonspecific interactions by tethering the antibody and MUC1 molecules to the atomic force microscope tip and sample surface with flexible polymer spacers. Rupture force magnitude and elastic characteristics of the spacers allowed identification of the rupture events corresponding to different numbers of interacting proteins. We used dynamic force spectroscopy to estimate the intermolecular potential widths and equivalent thermodynamic off rates for monovalent, bivalent, and trivalent interactions. Measured interaction potential parameters agree with the results of molecular docking simulation. Our results demonstrate that an increase of the interaction valency leads to a precipitous decline in the dissociation rate. Binding forces measured for monovalent and multivalent interactions match the predictions of a Markovian model for the strength of multiple uncorrelated bonds in a parallel configuration. Our approach is promising for comparison of the specific effects of molecular modifications as well as for determination of the best configuration of antibody-based multivalent targeting agents.