This study demonstrates a fast, immobilization-free way to measure monoclonal antibody titer directly in cell-culture harvest, using Flow Induced Dispersion Analysis (FIDA) as the central technique. FIDA, based on Taylor Dispersion Analysis in narrow fused-silica capillaries, was used to quantify rituximab (an IgG) directly in clarified CHO-cell fermentation broth with only about 4 minutes of analysis time. A small fluorescently labeled anti-IgG affibody served as the indicator: as it bound rituximab, its apparent hydrodynamic radius increased from roughly 2.2 nm toward a plateau near 8 nm. FIDA measured this size change across known rituximab concentrations to build a standard curve, characterize the interaction (low-nanomolar Kd, complex size, 1:1 stoichiometry) and reach a limit of quantification near 0.9 nM. Applied to two parallel CHO broths, FIDA-measured titers (~1.3–1.5 mg/mL) agreed with conventional protein-A purification and UV spectrophotometry.